Szilard Quates of Baccano!! routinely absorbs other immortals to gain scientific knowledge, and just 'cause he's off his rocker. All immortals have this ability, supposedly so that immortals who have grown tired of living have a way to commit suicide, but Szilard's the only one who uses it without consent in anything other than the most desperate of circumstances, even going so far as to make people immortal just so that he can absorb them in this manner.
Aaroniero can devour dead hollows, gaining their strength and powers, unlike other Arrancar/Hollows who only gain strength.
When Aizen was going through a series of level ups, he informed Ichigo that he was letting him live because he intended to return later on and devour him. Ichigo made sure that wasn't possible.
There is a being capable of embedding fragments of his soul into others, the fragment absorbs all the knowledge, experience and power of the victim, which he benefits from the moment he recalls the fragment to his own soul. By additionally carving an initial onto such a victim's soul, that victim becomes a means for him to spread his soul to anyone that person comes into contact as well. Recalling soul fragments kills the affected victims. Yhwach was born incapable of moving and with not a single one of his five senses functional. This power enabled him to gain the ability to interact with the world, but he has to keep doing it to avoid reverting back to his original state.
This was more or less the goal of Cell. He was created to be the ultimate fighter, but had to consume both Android 17 and 18 to assume his ultimate form. He later demonstrates the ability to learn the signature techniques of new fighters by absorbing their cells.
Majin Buu has the ability to absorb anyone, assimilating them into itself and gaining a measure of their intelligence, power, and... uh, fashion sense. However, it is eventually revealed that this had an unintended side-effect: giving Buu the sentience to regulate his power and use it in the pursuit of intelligent goals. When Goku and Vegeta free his victims, Buu reverts to his original form, lowering his raw power level to a still-very-significantly-high level but removing all of his moral restraints. He goes on to destroy the Earth just because it is there, wielding his full power like a blunt instrument.
Franmalth, one of the Nine Demon Gates of Tartarus, is a demon with this power in Fairy Tail. He can absorb people, body and soul, and claim their powers for himself. This renders him immune to magical attacks and direct contact physical attacks since he absorbs them too. His sole weakness is that he can't absorb something if it doesn't have a soul so Natsu beats him to death with a really big rock.
Shin Getter has shades of this, as its massive Getter Ray reservoir allows it to fuse with anything - at one point it combines with a nuclear missile in mid-flight. Its implied evolution, the Getter Emperor, is a much more straight example, at one point absorbing a planet by flying into it.
Alucard is another heroic example, for a given value of 'heroic'. Anyone he eats (and more importantly, drinks the blood of) has their soul absorbed into the totality inside Alucard, who can then summon them as a familiar and access their memories, form, and abilities.
Naraku from Inuyasha absorbs other demons into himself to gain power. He tried it on both Sesshomaru and Koga, as well, but it didn't go so well. Minor demons like the generic snake thingies, on the other hand, get sucked up like spaghetti. It gets to the point where he actually has to start cutting some loose because some take up space without adding much to his strength.
Katanagatari has Maniwa Houou. He can learn other people's abilities by grafting part of their bodies to his own. He did this to Maniwa Kawauso's arm and it's implied that he did the same thing to Emonzaemon's face. It is also implied that he could potentially gain immortality by repeatedly replacing his body parts like this. Additionally Maniwa Kyouken, who's ability is to take over other people's bodies might qualify. Once she gets a new body, her old body disintegrates to dust. She also absorbs the memories and fighting abilities of the woman whose body she steals and has inhabited 2,000 different bodies.
In Naruto, several of the villains fit this, most notably Orochimaru and Kabuto.
Tengen Toppa Gurren Lagann: In a very rare heroic example, the mecha Lagann is capable of assimilating other mecha into itself, resulting in the mecha of the series in the finale, big enough to use galaxies as shuriken. This initially appears to fail, as the Lagann just stabs into the cockpit of the Gurren and almost kills Kamina. It's only because of the Spiral Power of the pilots that it works later on.
Nrvnsqr (or Nero) Chaos is a vampire in Tsukihime whose main power is to incorporate the lifeforce of animals into his body, which is then made up of the chaos. He uses these animals to fight, and sends out progressively stronger and more legendary creatures as he his cornered by NanayaShiki.
From Creator/Marvel's more adult comics, there's The Terror, a living corpse whose body is continually rotting; he takes other people's body parts to rebuild himself, although these continue to decay. He gains their memories, skills, and sometimes even feelings; he has even been seen to give himself wings or a prehensile tail. He can even gain superpowers from the parts, although they have to be the parts that contain the powers (for example, he couldn't steal Cyclops' finger and gain optic blasts).
The Makuta from BIONICLE have the ability to absorb other beings to increase their mass but have to oppress the other minds to ensure that they would still remain in control of their fused bodies.
Green Lantern has Agent Orange, who absorbs the form and memory of those he kills, allowing him to create duplicates of them as subordinates. Having become the living embodiment of Avarice, he desires to own everything, including the very identities of his enemies.
Recent promos for upcoming Sonic the Hedgehog issues state that the Iron Dominion will be "legionizing" our heroes. What that entails is thus far open to discussion, but considering this comes right off the heels of the Dark Legion (who are allied to the Iron Dominion) being installed with computer chips that allows the Dominion and their leader Lien-Da to mindlessly control them remotely (the former using Magitek), and, well... just say that there's room for one more Star Trek reference, in the comic.
The aptly named Doctor Who/Star Trek crossover comic Assimilation2 is about the two most notable sci-fi versions of this, the Borg and Cybermen, joining forces to do this.
The Brood from X-Men reproduce this way, by injecting their eggs into unwilling hosts. This does not so much produce a juvenile hatchling as it does turn the unfortunate host into a Brood, which has all the memories and skills of its "parent".
The X-Men also have/had the Phalanx, who infected targets with a techno-organic virus and brought them into their machine-like hivemind.
The X-Men's own Rogue at times also qualified, as absorbing other people's memories and powers usually sent them into a coma. Cody Robbins never awoke from his.
The series explores the implications of this at one point, with a build up of all the evil absorbed from killing evil immortals eventually overpowering one guy, turning him evil, requiring some good immortal to defeat him (and thus absorb all that evil and become overpowered by it him/herself)
The Master Control Program, Big Bad of TRON, appropriates other (sentient) computer programs to obtain their functions. Clu in TRON: Legacy "rectifies" other programs, brainwashing them into becoming an army of Faceless Mooks.
In Wreck-It Ralph, the Cy-Bugs can incorporate anything they eat into themselves even characters.
The One in Animorphs, as well as Father from one of the prequels. (Coincidence? ...yeah, probably.)
The Vord in Jim Butcher's Codex Alera series has elements of this. They start out as a Hive Mind with a caste system; most of them are Insectoid Aliens ranging from the size of a dog to a bear, but some are much smaller and capable of serving as Puppeteer Parasites to animate and control people. This is a permanent condition and turns the victim into zombies. However, over the course of the series they learned the value of intelligent slaves, so they began using a version of the local Functional Magic to cause Happiness in Slavery so people would serve them while still physically alive and well.
Fate of the Jedi has Abeloth. Interestingly, when she absorbs Callista, Callista's feelings for Luke Skywalker cause her to fall in love with him too. This goes away when Luke manages to free Callista's spirit from her.
The horror novel Full Tilt takes a more impersonal approach to the trope: if you die in the Amusement Park of Doom, your face appears somewhere—in a cloud, on a rock, even on a billboard—with a smile on your lips but a scream in your eyes. If you don't get out by sunrise, but you survive, you're enslaved for the rest of your (un)natural life, but you retain your memories and a certain degree of freedom. And if you're really, really unlucky, you go to the Works. (It's never clear what precisely is taken, but the Big Bad says at one point that as the park grows in power it comes closer to being real and all other realities come closer to being imaginary).
"You will join me. You'll be a part of me. Didn't you want to become one with the Force? Isn't that what you told me? [...] The Force is nothing. If it ever existed, it belonged to Jedi who died years ago. I can offer you something more. Join me, and you will join thousands, millions of others. [...] You know, you're not strong enough to stop me. Not nearly strong enough. Once you're under my control, I'll make you my primary host. I will be you."
In Harry Potter and the Half-Blood Prince, Voldemort used Inferi, reanimated corpses, to guard one of his Horcruxes. These inferi overwhelm Harry and try to drown him. Harry suspects that he himself will become an inferi after he dies.
In the Nightside novel Agents of Light and Darkness, a minor character in the story named Belle does this. She hunts down powerful creatures, taking contracts on them if possible, and incorporates their skin into either her leather outfit or herself. She has a personal power letting her preserve and use the magic of the skin's original owners. (Her last acquisition before showing up was a pair of boots made of the skin of a minor Greek god with speed-based powers.) She lasts as long as most antagonists in these books: just long enough to explain in detail how dangerous she is before offhandedly nullifying her.
The Swarm from Wild Cards. Also, the Righteous Djinn.
In Those That Wake and its sequel, What We Become, the villains assimilate people, though in different ways.
The Sobornost mind upload collective in The Quantum Thief-trilogy seeks to absorb all existing minds into itself in hopes of eliminating all death and uncertainty from the universe.
The All-Defector, a monstrous and uncontrollable creation of the Sobornost seeks to go one step beyond and become the universe, due to its nature as a perfect mirror that becomes whatever looks into it, only (subjectively) better, and when it finds out about parallel universes it seeks to become all of them, as well.
The Source of All Evil in Charmed, who possesses his victims and assimilates their personalities into himself, becoming more powerful and intelligent. The problem comes when after assimilating Cole Turner, his feelings for Phoebe and humanity makes him difficult to control. Eventually it backfires, because he can't hurt Phoebe or her sisters without Cole interfering and making him vulnerable to being vanquished again.
The Krillitane from "School Reunion" alter their DNA with DNA from races they conquer.
The Abzorbaloff from "Love & Monsters" works on an individual level, absorbing individuals into his body.
Despite this always having been a large part of their characterization, the new-series version of the Cybermen do this somewhat more markedly than the Classic Cybermen "upgrading" people by converting them into more Cybermen now being their main shtick — this being the intention of their very human (well, until the Cybermen forcibly "upgrade" him into the Cyberleader) creator. One group of Daleks wasn't above using humans as raw materials in a similar way, either.
In the Eleventh Doctor episode "The Pandorica Opens", a mostly dead Cyberman's head actually says "You will be assimilated." The use of the phrase by the Cybermen in Doctor Who pre-dates the Star Trek use, as it was featured in the Cybermen's debut story, The Tenth Planet in 1966. It's likely an in-joke too. As River Song is examining transmitters in that same episode, the radio traffic of the Cybermen, which is audible in the background, sounds very much like the Borg Collective.
Sylar from Heroes, whereas Peter is more Mega Manning. Doubly so since Sylar can assimilate other peoples very identities, through a combination of a shapeshifting power that lets him copy anyone whose DNA he samples via touch, and a psychic power that lets him absorb memories via touch.
Debatable, since in "Best of Both Worlds" they do say "Your culture will be adapted to service us." While certainly their first appearance only reference technology they were after humans themselves the second time.
It could be that the Borg were not interested in assimilating humans in general, simply because they did not really need more drones and it was much easier to annihilate anyone resisting and then grab the rest after the defenders were killed off. In Star Trek: First Contact the Borg were noticeably short on man power so they were probably just grabbing everyone they could to increase their strength.
Dialogue between Locutus and Worf in Best of Both Worlds Part II indicates that the Borg do at this point intend to assimilate entire races.
Locutus: Worf, Klingon species, a warrior race. You too will be assimilated.
Worf: The Klingon Empire will never yield.
Locutus: Why do you resist? We only wish to raise quality of life for all species.
Worf: I like my species the way it is!
Locutus: A narrow vision. You will become one with the Borg.
In Deep Space Nine, it's shown that this is how some view the Federation. From their perspective, all they see is a bunch of Scary Dogmatic Aliens who travel the Galaxy, seeking out new life and new civilizations with the sole intention of eventually absorbing these new cultures into their own. After all, who wouldn't want to join the "glorious" Federation?
"You know, in some ways, you're even worse than the Borg. At least they tell you about their plans for assimilation. You're more insidious, you assimilate people - and they don't even know it."
Yes had a weird (but not evil) variant in all the bands it absorbed along the way, incorporating their musical styles as well. Yes absorbed New Wave band The Buggles, then "Cinema" (which was originally supposed to be a new band with Trevor Rabin as frontman), and then "Anderson Bruford Wakeman Howe", the last one being particularly notable as it was made up of former members of Yes, who were assimilated back into their own band.
The game Gargon is this plus Hide-And-Seek plus Tag!. Several people are chosen to be "It" and link arms, forming the Gargon, while everyone else hides. The Gargon looks for the hiding people, and when they're tagged, they become part of the Gargon. It lasts until everyone's part of the Gargon, or the Ref calls time.
This is what the Maelstrom is like in Dino Attack RPGat its best. At worst it's an unstoppable Eldritch Abomination that could theoretically become capable of destroying the entire universe.
The Hivebrood, an insanely complex Virus monster from the Basic/Expert/etc Dungeons & Dragons game system. Eating sentient beings lets them absorb their victims' know-how, spellcasting skills included. Don't know anything worth learning? Then they'll turn you into a Hive drudge-worker instead.
Another DnD monster with this kind of ability: the cadaver golem, which can take bits and pieces of people to absorb their skills. It's a Discard and Draw situation, though.
Played straight and subverted with the Illithids (aka Mind Flayers). They reproduce by putting an illithid tadpole into a victim's ear, which consumes their brain and turns them into an illithid. Subverted in that the new mind flayer does not retain any memories or skills from the host. Usually. They might sometimes tap their fingers in a particular way, or hum a tune the original person was familiar with. Doing so is considered a very bad sign, and they avoid it when possible. Played straighter with a Prestige Class that Mind Flayers can take that allows them to absorb skills and powers from the creature's whose brains they consume.
There is also a drug that a host can take that offers a chance (no guarantees) that they will retain their identity and free will after being transformed, allowing them to keep their knowledge and skills as well.
Formian Dominators are charged with bringing under Formian power anyone who is unwilling to join the Formians peacefully. Each one can control four or five beings at once.
Jedah of Darkstalkers fame is on a holy quest to unite all souls into himself in order to stop the endless wars engulfing both the Demon Realm and the human world. He often uses phrases like "Now, you are a part of me."
In Final Fantasy IX, one of the evil plots our heroes have to foil is that Garland is trying to assimilate Gaia into Terra in an attempt to avert the death of his world. And it turns out that Kuja, previously thought to be the Big Bad of the game, was actually working for him in starting wars across Gaia.
Just to hang a lampshade on it, the most recently introduced armour upgrade for the Blue Mage is the Assimilator's Attire.
In Guild Wars, it's the only way to acquire Elite skills: find the boss, beat the crap out of him, apply the Signet of Capture.
There is every indication that The Combine from Half-Life 2 and its Episodes do this to the planets/universes/borderworlds they conquer, most obviously with their Synth units (gunships, dropships, Striders and Hunters). They appear to be in the process of doing this to humans, with designs for far more altered human soldiers created in production but removed from the final game. They also assimilate and adapt technology that they find: they're on Earth specifically for our teleportation technology perfected at Black Mesa. It's also implied that it's actually a favor to those who surrender; they are powerful enough to exterminate anything that gets in their way, and you need to convince them that your species is more use to them alive and enslaved than dead. That's essentially what Dr. Breen did to stop the Seven Hours War.
It's revealed in Kingdom Hearts 3D [Dream Drop Distance] that this was the main reason Xemnas founded Organization XIII. Nobodies, having no hearts, make perfect vessels for the disembodied Ansem. In fact, had everything gone according to plan, Xemnas could have turned half the Organization into duplicates of Xehanort.
This comes after a rather cruel reveal. It turns out that nobodies quickly develop new hearts from whatever is around them, and CAN have emotions. This incredibly hopeful reveal is immediately quashed by Xemnas revealing the above plan.
The Reapers from Mass Effect. Every fifty thousand years, they "harvest" all life in the galaxy to turn into more Reapers.
Harbinger: We are the harbingers of their perfection!
Also the Asari, a rare non-villainous example. When they mate with anybody, the offspring will still be Asari.
Mega Man. Once a robot master is defeated, all iterations of Mega Man gain the boss' skills.
The X parasites in Metroid: Fusion absorb DNA from anything they infect in order to copy, multiply, and spread.
Osmos is about controlling a primordial cell in a colony of others like it. Your goal is to absorb as many of other cells as possible and/or some special cell in question. Of course, other cells will also absorb anything small enough in their way: the difference is that they can’t steer themselves around.
Kyurem is revealed to be this in Pokémon Black 2 and White 2, where it turns Reshiram and Zekrom into powerups for itself that allow it to change form into Black/White Kyurem. To be fair though, the three dragons used to be one. Kyurem just wants to be whole again.
It helps that as he picks up their memories, he gets a bit of their personalities as well, slowly going from the completely remorseless sociopath he was before he got killed to simply a very bad individual.
With Alex as the Big Bad of Prototype 2, James Heller, the new protagonist, shares many of the same attributes as Mercer did in the first game, though he's a better person from the start.
Morphius from Return To Zork, is in the habit of looking for new character traits he can incorporate into himself, often watching potential subjects through the eyes of vultures or manifesting himself in their dreams. If he finds anything of value, he has the vultures carry them off to his Citadel, where he turns them to stone and goes about assimilating their characteristics. By the endgame, his eyes are on the player character - specifically for your strategic gifts.
Like the above Tyranids, the Zergalso want to use your genetic material for its evolutionary advantages and for raw material. The effects of this become more apparent in Starcraft II, where the Queens take on a more human-like (or possibly protoss-like) appearance.
Most Festum in Super Robot Wars UX can assimilate robots by doing this. If Morale is reduced to under 90, the unit they attack is assimilated and destroyed. Festum however do not drain very effectively, the Morale you gain from taking hits and counter-killing Festum largely ameliorates the morale lost from an attack. The Mark Sein can also Assimilate but much more effectively. Kazuki deals severe morale damage when he attacks a festum and he assimilates enemy Festum if they're reduced below 120 Morale (Even if they didn't have that much to begin with).
In Sword of the Stars, Zuul Mind Rape removes prisoners' memories, knowledge, emotions, thoughts, and eventually their very identities. The Zuul that performs the Mind Rape absorbs these things for themselves. As the Zuul are connected by a telepathic Hive Mind they do not have much in the way of a 'self', and therefore finds things like a name and individuality to be extremely valuable.
The Many in System Shock 2. They’re the ultimate evolution of the biological experiments bred by SHODAN in the first game and have a complete Hive Mind and are intent on absorbing everyone else. Throughout the game, they will even calmly urge you to join them, contrasting SHODAN’s cold metal with the “attractive” warmth of their combined biomass.
Even so, they’re not above creating cyborgs to better protect their hatchlings. The half-skeletal Cyborg Nannies, fitted with laser weapons and created from ship’s nurses, are some of the most difficult and disturbing enemies in the game.
The Alien Death Slug from The Visitor. It eats everything it comes across to absorb their DNA, and uses it to rapidly evolve from a worm to a bizarre mashup of features.
In World of Warcraft, the Horde faction, the Forsaken were originally undead who managed to break free from the Scourge, a huge faction consisting of cultists, undead, and necromancers. After the Scourge's leader was destroyed, the Forsaken's leader, Sylvanas, essentially hired some of the Scourge's old minions who were capable of raising undead. While some of those that were raised by the Forsaken were free of mind like most of them, in warzones, she would raise enemy troops to be unquestionably loyal to her and force them right back onto the battlefield for her side. This became so bad that by the time the PC would join the conflict, humans from the opposing side refused to join battle, leaving only non-human races and humans who decided to become werewolves to fight the Forsaken since only humans can be raised as forsaken.
And he says he did that in his universe with one of the musicians at the Figrindan orchestra.
MSF High has the Legion, who are unique in this in that they're actually good guys nowadays. A big plot point is that Legion retain characteristics from the old life, or are exactly the same (however, they've still been compared to both the Borg and the Flood). They do have a hive mind, however.
The Akinator will successfully guess the character you're thinking about, then use your answers to add to its own knowledge of this character. Or if he fails to best you, he'll still increase his knowledge anyway.
This ends up being the goal of Missingno during The Entity arc of Atop the Fourth Wall, to absorb all existence into itself.
Mr. Popo does this to Blue Popo in Dragon Ball Abridged in a video advertising their visit to Youmacon 2010.
Mr. Popo: Okay, I'll bite, what the hell is this?
BluePopo: Oh. Hello, my crass counterpart. I'm Blue Popo, and I...
Mr. Popo: No.
Blue Popo: Pardon?
Mr. Popo: Prepare to be assimilated.
Blue Popo: Whatever do you me... [black tentacles infiltrate his blue color] No! No! No! Kami!
In Ben 10: Omniverse, Malware is a mutant Galvanic Mechamorph (a being of living technology), who can permanently absorb other devices in order to upgrade himself, as opposed to the rest of his kind, who can upgrade existing technology (though it is implied Malware can do this as well, but chooses not to). Before an upgrade, however, he caused anything he absorbed to corrode, even OTHER MECHAMORPHS!
In the original Ben 10 series, we have Dr. Victor, who can take control of technology under certain circumstances (justified in Word of God info, as he is revealed to come from a Frankenstein monster-like race).
The Brains in Futurama seek to copy and store record of everything, thus preventing anything new from happening and having complete, universal knowledge.
Cyborg!Trina in the Grojband episode "Ahead of Our Own Tone".
Alpha from Men In Black: The Series used to provide the page picture. He was the original head of MIB but now hunts aliens for body parts to make himself more powerful. The first time we see him, he's still humanoid but has a small arsenal of Combat Tentacles concealed in his body, and he only gets freakier from there. Interestingly, he moves away from this trope during the last season, modifying himself using robot parts instead.
The Game Master from Teen Titans wanted to assimilate the gathered heroes' various powers. He manages to absorb all but Robin; who ironically has absolutely no powers and thus nothing to give him. He then proceeds to have his butt handed to him on a platter.
"I remember you... EMP generator, right? I'm not good with names and faces, but I never forget a trophy."
In the tie-in comic, it's revealed that his payment includes parts from an associate of the Decepticon who hired him. That's right, he wants parts from his employers too.
The Spirit Drinker in X-Men seems to be this, though it could just be a hungry beast, and once the souls are fully consumed, they're just gone. The Phalanx, on the other hand, do assimilate people. In fact, the animated version actually said "Resistance is futile; you will be assimilated."
Retroviruses directly integrate their own genes into the genetic code of a host cell in an attempt to turn it into a factory for more viruses, which sometimes inherit DNA from the original cell. This actually contributes significantly to the speed of evolution, and some of the "junk" DNA originated from retroviral infections.
An unfortunate characteristic of nationalism in many countries in the past. Virtually every country, usually with force, has behaved this way toward ethnic minorities in some manner. To what extent remains a very contentuous subject even now.
The Falcon programming language is this trope applied to language design. It purports to have integrated six different paradigms, and seem to have the overall philosophy that "if it exists, we want it".
C++ does the same thing, although it's attracted something of a hatedom for it (in fairness this is because several of the features it has absorbed are mutually contradictory). Programming languages tend to be criticized for doing this because it can end up impossible to use all of the language to do what you want, or to constrain and focus your thought process.
Besides trying to murder every single Jewish person they could find, and to turn Eastern Europe into a German colony and the Slavs into a Slave Race, this trope is essentially what Those Wacky Nazis wanted to do to the "Teutonic" states of Western and Northern Europe (Norway, Denmark, the Netherlands, etc.): force their inhabitants to accept the Nazis' racist dogma and to integrate them directly into the totalitarian society of the Third Reich.
This is a basic tactic of any military empire. As an empire grows, its army must grow to defend the new land, and the only way to do that is to get the people you just conquered to join your army. This allowed tiny backwater settlements like Rome and Macedon to conquer the known world simply by doing it one nation at a time and getting a larger army every time. Some of the greatest generals of all time, such as Pyrrhus of Epirus and Hannibal, won every battle they fought but ended up losing the war because they couldn't get the allegiance of the local people.
The extent of this depends on the empire. While the Romans might have assimilated the people their own attitudes and culture changed little. Compare that to the Normans who went from (essentially) mead drinking Vikings to wine drinking Christians in a generation, became the ruling class of a hefty swathe of the world and ended up being both among the most enthusiastic crusaders at one end of Europe and on such good terms with their Muslim citizens at the other that the most extravagant cathedral of the era boasted that its decoration had been done by skilled Islamic craftsmen. The Romans assimilated the people, the Normans assimilated themselves.
What happened to the Normans also happened to the Mongols: they conquered vast swathe of the world and end up inheriting the culture of the conquered people. This happened in China, Central Asia, and the Middle East. The Khanate horde was eventually dissolved, since the now-settled Mongol-descendants are no longer interested in obeying the words of some tent-dwellers from half a world away.
The United States of America. The national motto of "E pluribus unum" means "out of many, one"; the Statue of Liberty says "give me your tired and hungry, your huddled masses." Only instead of going out to aggressively incorporate other peoples (early days notwithstanding), citizens of other nations are invited and encouraged to add their culture to the national gestalt. Hence, "the Melting Pot." Inevitably, there are some who don't agree that this is a wholly good and desirable thing.
The European Union boasts a similar moto (In varietate concordia "United in diversity"), and since its inception 22 states (23 if you count East Germany) joined the 6 founding members in a -so far- bloodless process of assimilation of new territories and people that multiplied its population by three and it's surface by four. Just like the US, the assimilation process (called "enlargement") is a subject of much disagreement and debate among Europeans
This wiki. It doesn't matter if you stopped in here for a moment just to read a couple of pages on a lark; eventually, you're going to be tempted to add an example to one of them. And then another. And another. And another. Before you know it, you'll have launched a dozen tropes through YKTTW, created at least one page for a film, book, or series, and participated in one debate where ninety percent of the conversation is entirely in Wiki Words. But don't feel too bad. If it likes you, it rewards you by pushing you towards new media in which you previously had no interest. As masters of your time and attention go, TV Tropes is kind.
This was also a defining characteristic of Habsburg Austria until the end of World War I, which often expanded and consolidated its territories through diplomacy, marriage and conflict.