Superheroes, Japanese style. The word can be translated as "Task Force" or "Squadron." Usually come in teams, with color-keyed uniforms and a range of personalities/roles that usually follows some variation on the Five-Man Band. Known for their synchronized posing; also, a Standard Powerup Pose is often used in many Sentai works during the Transformation Sequence. A key concept is that the collective team are more important than the individual, and that more can be achieved by working together.
Although popular in anime, there have also been innumerable live action Sentai series in Japan, most notably Super Sentai, the series which was re-edited into Power Rangers in the United States, and 2003's Pretty Guardian Sailor Moon.
A very western form of Sentai is to feature five teenagers receiving powers, with a transformation, i.e. it doesn't have to involve color-coding, or full body suits. They may not even transform at all, but have powers that combine to make a greater whole.
Contrary to popular belief, "Sentai" ONLY denotes to shows that have a squad of nigh-identical, color-coded superheroes, not every transforming superhero that comes from Japan (the proper term for that would be Henshin Hero, that belongs in the Tokusatsu genre).
Do not confuse with Hentai, please. Even though there's definitely some unholy overlap somewhere.
Saint Seiya is a classic anime mixing Sentai with mythological fantasy.
Mahou Sensei Negima!: In an obvious parody of Super Sentai/Power Rangers, the five girls from Negi's class with the lowest grades dub themselves the "Baka Rangers," complete with appropriate "hero names," like "Baka Red" and "Baka Pink". For clarification: Red = Asuna, Blue = Kaede, Yellow = Fei, Black = Yue, Pink = Makie.
During the festival story arc in the manga, there is the "Maho(ra) Rangers" performing on a stage.
Keroro Gunsou does a few parodies of the Sentai genre. Episode 24 introduces Space Detective Kogoro, an explicit parody of Kenji Ooba in his role as the title character of the Toku series Space Sheriff Gavan.
Spoofed in Mai-HiME, where Midori (the one who Jumped at the Call, thinking she's Sailor Moon) proposes teaming up into the "HiME Rangers" to fight the monsters of the week more effectively. The two HiMEs who dismiss the idea as ridiculous ( Natsuki and Nao) are tied up and dragged to the meeting place anyway.
Chouken Sentai Blade Braver in Bamboo Blade. Tama, the main girl in the show, considers herself an "ally of justice" like the Blade Bravers themselves, and in fact uses the motivation of fighting for right to join the kendo club. Later on, the show reveals that Braver is just one of many in the 31-year long "Battle Hero" series, making it an obvious Lawyer-Friendly Cameo of Super Sentai.
The Ginyu Force of Dragon Ball Z are a parody of this trope: there are five of them (instead of the four which are traditional in villainous groups), and they constantly strike poses and throw group attacks. In fact, when recruiting new members for the team, Captain Ginyu is mostly concerned with their posing ability.
Magical Pokaan has a singular, Sentai-based episode that becomes increasingly silly. Since the show only has four leading girls, the fifth spot is filled by no one — the green suit is an empty shell. On top of that, the team's Combining Mecha never forms correctly, with mishaps ranging from a missing component to five copies of the same component attempting to fuse together. The end of the episode is a stream of random gags that would take too long to explain here.
There actually is a fifth ranger. She just happens to be invisible.
The Excel♥Saga manga is somewhere between a Deconstruction and Parody of this genre. The Municipal Team Daitenzin is Sentai group made up by the local government to fight the evil organization ACROSS, but it's staffed by a collection of disinterested or stupid employees who do more damage than they fix.
The anime, meanwhile, throws out the serious elements and goes into full-blown parody with Municipal Team Daitenzin.
School Days' OVA Magical Heart Kokoro-chan has elements of this and Magical Girl in it. In this particular instance, the team of powered heroines is working for low pay, using a vacuum cleaner as a weapon and has particularly kinky outfits for their battling.
Parodied and deconstructed in Franken Fran: Takeshi, a sprinter with a bone disorder, goes to Fran and receives surgery to rebuild his body. Unfortunately, several other sprinters received the same surgery — when they all break the world record, he's discredited and disgraced. The others beat the crap out of him, so he gets more surgery and becomes a sentai hero named Sentinel. Then he becomes a Knight Templar, beating people up for littering, until finally his victim's family members gun him down. He receives one more surgery, leaving him monstrous... and in his new form he is attacked and killed by more sentai. The end of the chapter shows the city in a riot of Let's You and Him Fight battles, while Fran is complaining that everyone wants superpowers but nobody wants to pay for them.
Most shows about Combining Mecha use the sentai style to some extent — in fact there's even a Combining Mecha anime called Chojin Sentai Balatack, which includes the Five-Man Band with color-coded outfits. About the only thing these sorts of shows don't use is the "Super Sentai" Stance, since typically the heroes rarely fight outside their vehicles or robots, but even so, the combined robot will usually do plenty of posing upon combining.
Bleach episodes 212 and 213 parody this like there's no tomorrow.
One of the earliest strips in GA Geijutsuka Art Design Class has the five lead girls becoming the "Color Rangers". Keeping with the art theme, they're yellow, magenta, cyan and monochrome. Except for Tomokane, who's red, because every good Sentai team needs a red leader.
Another Show Within a Show example is the Star Rangers in Muteki Kanban Musume. Akihiko is a huge fan of them, to the point that when anyone so much as mentions the show, he starts to think he is Red Star. He seems equally convinced that the rest of the main cast are the other Star Rangers.
Kankuro:How are you Akihiko, are you okay? Can you walk on your own?
Akihiko:No. I don’t think so.
At Akihiko’s house, he is watching Star Rangers, with his arm and head bandaged: Star Ranger’s narrator:After destroying the Evil Base, the Star Rangers put hope back on the young boy’s heart. Today, they fought for justice yet again.
Akihiko: Dramatically standing up adopting an Ass Kicking PoseDon’t forget to watch next week!
The Teenage Mutant Ninja Turtles, who already shared superficial similarities to Sentai, were turned into this outright for the Japanese OVA Mutant Turtles: Legend of the Super Mutants—complete with the gigantic super-form "Turtle Saint".
In the Fruits Basket manga, Kakeru seems determined to turn the student council into a proper "School Defense Force" and starts assigning colors. He claims Black for himself (because he thinks it would make him seem cool and mysterious) and, after teasing Yuki about how his pretty face should make him Pink, gives him Red for being student council president. Kimi insists on being Pink and sticks Machi with Yellow (like Beat above, Kimi claims it's because Machi likes curry so much). Naohito just plain refuses to play along. And Ayame is named the Commander immediately after Kakeru meets him, much to his delight.
Angel Blade, once the series added enough girls, ended up becoming...a Sentai Hentai.
Mitsudomoe has the Show Within a ShowHonki Sentai Gachiranger. As Mitsudomoe is wont to do, the show seems a little more sexually charged than usual for a Sunday morning TV show. Hitoha is a huge fan.
Also spoofed in Yumeria: once all the girls are active in the dream world, they look at their individually-colored outfits and their attacks, and decide they need attack names and a victory pose.
Level E has a whole story arc of the anime version devoted to several kids who were pranked into becoming a Sentai team because... well, The Prince felt like it!
Jewelpet Kira Deco subverts this by stripping its titular (and very over-the-top) sentai team of their powers after they enter Jewel Land in episode 2. Each member does his/her own thing for the majority of the series instead of doing... sentai things.
Green Lantern has picked up some Sentai traits with the "New Guardians" teams, comprised of members from seven color-coded and Personality-Powered Corps. None of these people are much for striking poses, but they have the shonen speeches down cold.
The Handsome Men in killer7 are an antagonist version.
Parodied in The World Ends with You's bonus chapter, "Another Day." After everyone's pins get stolen at a Tin Pin Slammer tournament, Shooter brings Neku and his friends to his "secret base" (really Ramen Don) to form the Tin Pin Rangers and save the day. Shooter calls Red for himself as team leader, Beat gets Yellow because of his love of curry, Neku gets assigned Black n' Blue, much to his annoyance, and Shiki gets Green to match her skirt... but only because Joshua called dibs on Pink. Rhyme joins the team later on and becomes Black, leaving Neku with only Blue, until she leads them all into a trap and reveals herself as the Sixth Ranger Traitor.
Star Ocean: The Second Story: In a Private Action with Leon in Fun City, the "Scummy Rangers" show up in a deliberately over-the-top show where they dispatch evil monsters with the help of the kiddy audience.
Culture Brain intended to bring this to the US under the guise of a superhero team by converting two of their Hiryu No Ken games into one superhero-themed title on the NES, calling it Flying Warriors. They even had a fairly big marketing blitz to go with it, with a multi-issue comic book being published in Game Pro. The game itself ended up being swiftly forgotten.
Spoofed in God Hand with the Mad Midget Five: a squad of five characters in colour-coded outfits with high-pitched voices, none of whom even reach the main character's waist, prentending they're super heroes and pulling flashy moves. Add to that their high speed and agility, and you have one of the most annoying boss encounters in the whole game.
Persona 2 brings us Phoenix Ranger Featherman R. They're cliched, but not a parody. And the heroes of Innocent Sin count, as they roleplayed as these heroes in their childhood, and you need all five masks to unlock all five ultimate Personas.
Phoenix Ranger Featherman R makes a return in Persona 3. Each week you can catch the "next episode" bit on the television in the dorm's lounge.
Featherman is Yuuta's favorite show in Persona 4. P4 Golden allows you to buy Neo Featherman costumes that the heroes can wear while dungeon crawling. Party characters will even perform a "Super Sentai" Stance after wining a battle.
After the events of The Answer in Persona 3, Yukari decided to become an actress in Phoenix Ranger Featherman R. The result is the outfit and bow that she uses when she becomes playable in Persona 4 Arena.
Mentioned/Parodied in Tales of the Abyss with the "Abyss Man" costumes for the seven playable characters (if you've already beat the game once). Everyone gets a colour-coded costume (red, orange, blue, green, pink, black, and silver) with a hilarious description. "Abyss Man" is apparently a popular show in-universe, and the Emperor of Malkuth is a huge fan. Considering the two lines given that are evidently direct quotes from the show, it's a parody of the genre.
American animators have made their own version with a twist: Super Robot Monkey Team Hyperforce Go! features a Sentai-ish team of, well, robot monkeys, complete with different colors and personalities.
Captain Planet and the Planeteers carries much of this concept. Instead of transformations, they use powers from their elemental rings, and combine their powers to call Captain Planet.
Teen Titans also makes reference to sentai with the team colors (Beast Boy: Green, Robin: Red, etc) which are displayed at the end of its upbeat Title Sequence, then of course there's all the team poses.
Parodied in Kim Possible with Team Go, consisting of Mego, Hego, Shego, and the Wego twins.