Until the abolition of the samurai class in Japan in 1868, a ronin ("wave man") was a samurai without a master, generally through the former master's death, the individual ronin's disgrace, or the destruction of his lord's clan. Without a job, an income, or a home, ronin could and would spend their days Walking the Earth looking for a master willing to take them on. It was common for ronin to take positions as yojimbo (bodyguards). Others resorted to banditry or organised crime.
Although in ancient days ronin were considered a dangerous threat since it was believed they were likely to become bandits, the noble ronin is a common hero type in Japanese pop culture, typically acting as a Knight Errant.
In the modern era, ronin has two additional, different meanings: either a student who has not yet passed his college entrance examinations, but continues to study and try for a place or a salaryman who is temporarily between jobs.
See also the film Ronin or the comic maxi-series Ronin by Frank Miller.
Compare Street Samurai, Samurai Cowboy. Compare Dangerous Deserter for the less romanticized (and not necessarily Japonesque) version. A rough western equivalent would be the Black Knight (of the Real Life mercenary type, not the Tin Tyrant type.)
The most famous example would be the legend of the "Forty Seven Ronin," a band of samurai who took revenge for their master's death, and were allowed to commit seppuku for their crimes to keep their honor rather than be executed. Though based on a true story, it has seen many changes over the years. Considered a shining example of loyalty, honor and bushido, and the best-beloved story in Japan.
Neal Stephenson's The Baroque Cycle: Gabriel Goto. He's the son of Japanese Jesuits exiled from Japan when Christianity was banned; he's a fervent believer in both Catholicism and bushido. He eventually gives his swords to Jimmy and Danny Shaftoe, who later end up traveling America as the "Red-Neck Ronin".
Final Fantasy X: Auron has a lot of ronin qualities to him, such as the "death of a former master" thing, what with Braska dying at the hands of his own Final Aeon, Jecht. He also has three other major features used to signify a ronin: his left arm is held tucked out of his sleeve, he keeps a gourd of alcohol on his belt at all times, and his overdrive category is named 'Bushido'.
Kazemakase Tsukikage Ran: Ran is an uncommon example — a 'female' ronin who's actually a much better swordsman than her male colleagues. While she never resorts to banditry, she often scours the roadside for lost wallets or change, or tricks/charms her companion Miyao into giving her money or a free meal.
Miyamoto Musashi in most depictions of him. Justified as Musashi spent most of his life as a ronin, and tended to go in and out of service to various lords and patrons. Mitsurugi of Soul Calibur is heavily based on him.
Rurouni Kenshin: Kenshin Himura, except for the fact that he was never a samurai. "Rurouni" actually means "wandering swordsman", which is a more accurate description than "ronin".
The Seven Samurai were ronin. Except Kikuchiyo, who was born a farmer's son.
Samurai 7: The anime remake of Kurosawa's classic film.
Samurai Champloo: Jin and Mugen both have characteristics of ronin, though only Jin is actually a ronin. Having grown up in a penal colony Mugen was not born into the samurai class. (Nor did he earn his way with his skill as he used those skills for stealing from the shogunate).
Shogun's Samurai: The tide turns in a civil war between two brothers vying to be shogun after the supporters of one brother frame the other for hiring an army of ronin to kill the first brother.
The constant civil wars ensure that a steady stream of them pass through the tiny fishing community in Onibaba.
In the Morgaine Cycle books by American author C. J. Cherryh, the main character, Vanye, is an ilin, which is more-or-less completely analogous to ronin. Exiled for fratricide, he must Walk the Earth, and if he ever takes succor from a Lord he must serve them for a year and a day in recompense.
Usagi Yojimbo by Stan Sakai. A perfectly named comic as it means "Rabbit Bodyguard". This is exactly what the main character is.
In the WALLE Forum Roleplay, the Japanese Autopilot KATANA insults people and robots whom she sees as disgraced samurai who turned their back to the Way of The Warrior, or otherwise failed their Daimyos (note: she means superiors), as ronin.
JAXA, another Japanese Auto, on the other hand, embodies the trope of the samurai without a master who became a dangerous bandit.
Age of Empires III has two different types of ronin. The Ronin from the classic game are heavily armored fighters who can only be recruited from the home city as part of a mercenary band. The ronin from the expansion are much weaker, lightly armored and closer to the concept of wandering swordsman. Both types of Ronin are recruitable if you choose to play as Japan.
Kat and Mouse: Guns for Hire features two ronin from the future named - Kat and Mouse.
In The Water Phoenix King, we find the non-human Ngapp, who supported the titular God-King in hopes of supplanting his rebellious human subjects, and were retroactively outlawed after their emperor decided to support the human resistance at the tail end of the war. Now exiled to the wastelands, men without a country or hope of pardon, they have turned bandits and raid against neighboring humans and their own people, the Yigs, indiscriminately. Recently one of these — a Sorcerous Overlord who was originally just an Alchemist looking for the scholarly respect denied him at home — has found a powerful, if secretive, backer and is starting to unify all the separate Ngapp bands under his aegis, posing an increasing threat to neighboring kingdoms and principalities on all sides...
Zero Takaichi of Tasakeru is a ronin of the "disgraced samurai" variety.
Hirayama and Sahara from 13 Assassins. Hirayama has worked as a mercenary for years, but owes his allegiance to his former sensei in swordfighting, while Sahara joins the main characters' plight because he finds their cause worthy, and in exchange for a payment.
Lupin III has Goemon Ishikawa XIII; a traditional samurai in the modern world. He constantly seeks to improve his skills, seeking teachers of Martial Arts (whom he then refers to as his "Master", rather than the more traditional "teacher"), as well as operating as a hitman or Yojimbo. Everyone In-Universe calls him Samurai, but he is a Ronin by our definition. Goemon also believes that his attitude is normal, and that most of Japan has lost their way, instead.
The player character in each Way of the Samurai game and a lot of supporting cast as well. Depend on the choices made by the player, dependent upon the game but some of the most common boil down to, join up with a new master, join (possibly noble) gangsters, continue to wander aimlessly as the game goes by doing odd jobs for money, or support a third "civilian" side in the local conflict.
Anpanman has Omusubiman and Komusubiman, two rice ball-headed characters that travel througout the village, doing random good deeds when needed. Komusubiman's still a kid, so he still has a lot to learn from Omusubiman.
The prosecutor Simon Blackquill from Phoenix Wright: Ace Attorney - Dual Destinies has a very strong ronin theme, constantly using sword metaphors, wearing a coat that resembles a jinbaori, and occasionally casting Razor Wind with his fingers in lieu of a real blade. He was even disgraced after the death of his mentor Metis, due to being falsely accused of murdering her. He spent seven years in prison and came within a hairsbreadth of execution just to protect Metis' daughter.
Ichiroh! has its protagonists Nanako and Akane. In fact, the title is the term used for someone in their first year as a ronin.
Saya of Servant × Service spent a year as a ronin under this definition. This is why she's the oldest of all the newbies.
Real life example: In medieval China, the only way to enter the civil service was to take multiple, incredibly difficult exams. Pu Songling took his first test at 19 years old and succeeded, but got no higher until he was 71. In the meantime, he collected folk tales and mystic stories, which remain with us now as his Strange Stories From A Chinese Studio.
In Weekend at Hisao's, Takumi, Hisao's old friend, uses the term's variant for college students to describe himself (the fic takes place after he and Hisao graduate from high school), as he failed to get into the school that he wanted.
Other uses of the term Ronin
Kingdom of Loathing uses the term for the state inflicted after one ascends and starts over. While in Ronin, they can only access a limited number of items or money from their previous life, and they can't receive things from other players. Ronin expires after a set number of turns, unless you're in Hardcore mode, where it doesn't expire until you finish the game again and you can't access your previous life's items at all.
Two Marvel Comics characters - Maya Lopez and Clint Barton (better known as Hawkeye) - have used the name "Ronin". It's basically a placeholder identity for heroes who can't use their normal superhero identities for whatever reason. Yukio from Frank Miller's Wolverine does not use it as a name, but she is actually described as a ronin.
The film Ronin is set after the cold war, and compares former secret agents to Ronin.
Saints Row 2: There is a gang called the Ronin. Although they seem to be Samurai, they have 2 masters: Shogo Akuji and Kazuo AkujiLampshaded by NPC chatter: "Why are we called the Ronin? We have a leader!"
Stargate Atlantis: Though never a Samurai, Ronon Dex was a soldier defending a world culled of sentient life by Wraith, and one of the few survivors. His name is also a play on the word Ronin.
The X-Files: "Pusher" Modell considered himself one, acting as a hitman and making many references to Japanese culture.
The main character's team in Unreal Tournament III's singleplayer mode is called Ronin. Reaper acknowledges the meaning when his team is drafted by the Izanagi corporation, commenting "So, the Ronin have a master."
The titular Ronin Warriors are an unusual case in that while they do lose their master, the name is given from the get-go. Given that in the original Japanese they were called the Yoroiden Samurai Troopers, this is most likely a case of Rule of Cool.
In Werewolf: The Apocalypse, Garou who don't belong to any tribe are known as Ronin, and are pariahs in a setting that places heavy emphasis on community and group dynamics. Interestingly, they are not a popular choice for players with Special Snowflake Syndrome, since their "tribal" weakness — an extremely difficult time gaining renown — is much harsher than those for the tribes. For added Bilingual Bonus, "Ronin" can also be read as "Wolf Man."
In Command & Conquer: Red Alert 3: Uprising, Steel Ronin are a kind of battlesuit unit. Specifically, they are in the suits as a form of punishment for not following orders or related crimes and hope to regain their freedom again by doing battle against the Empire's enemies. They're apparently in constant pain too. Meanwhile, in the Game ModRed Alert 3: Paradox, Steel Ronin have layed down their weapons instead and, among other things, became tourist guides due to being in constant pain as they figured "Why should we follow these people again?"
Person of Interest. The POI in "Wolf and Cub", who's a fan of samurai films, compares Reece to a ronin. Given that Reece is a former CIA assassin turned Knight Errant, he's not far wrong.
In the film Epic, the Badass leader of the Leafmen is called Ronin, foreshadowing the death of his queen early in the film.