A narrative technique in which we're shown events that took place before the episode's main action. Said events may have taken place on-screen earlier, but may also be new scenes depicting things hitherto only referenced.
Specific types of flashbacks include:
The Tale Of One Bad Rat: Flashbacks are used several times in the story, especially the first half, to show significant events in Helen's life before she ran away. One example is how she adopted her pet rat.
In the Total Drama story, Legacy, chapter 2 and most of chapter 3 cover events from years before the main setting, but are told as if they were set in the same time frame.
Chapter 18 of Super Paper Mario X starts with Sonic, as always, scared of going in the water (since he can't swim and will drown in the water, after all). When he is informed that this was not the first time the group had to go through water, Sonic says to "name the times", followed by a flashback to the first Paper Mario X (lampshaded by Mario). When Sonic says there was no way it happened twice, a flashback to Paper Mario X 2 follows.
In Call of Duty 4: Modern Warfare, when Price finds out Zakhaev is behind the game's plot, he decides to roll a flashback to explain the guy's background. Players know this as the "All ghillied up" mission.
Maji De Watashi Ni Koi Shinasai has flashbacks interspersed between routes as necessary; no single path gives a complete background on the family, with each person's pertinent set of flashbacks happening in their own storyline.
Concerned has an mini-arc when Frohman flashes back to his days at Black Mesa, providing arguably the best Lampshade Hanging of this trope ever as the quote for this page.
Invader Zim also lampshades a common problem with this trope: In "The Fry Cook What Came From All That Space", Zim recalls a flashback of being demoted to fry cook under fry lord Sizz-Lorr, and then escaping. After Zim escapes, it shows Sizz-Lorr alone, shouting at the top of his lungs:
Sizz-Lorr: I will find you Zim, so help me, I will search the entire universe, and I! WILL! FIND YOOOOOOOOU!
(Cut back to present time with Zim and Sizz-Lorr)
Sizz-Lorr: How did you remember what I said if you weren't there?
Used a lot in Phineas and Ferb by the character Dr. Doofenshmirtz, since almost all of his inventions have a backstory and he uses flashbacks to let Perry and the audience know the purpose for it. However, there have been some occasions where he'll skip over a flashback since Perry already knows the story, and on one occasion he had a horrible headache and said it hurt too much to do a flashback.
In "The Belly of the Beast", Perry was going into a flashback of how he escaped, which Doofensmirtz took advantage of by attacking him.
Parodied in "Doof Dynasty", an Elseworlds episode set in ancient China, where this discussion occurs about "Master Perry".
Phineas:...does he know we can't see it? Should we give him some privacy? I don't know the protocol for flash-backs.
Used frequently in Avatar: The Last Airbender, usually in the form of someone in the present narrating something that happened in the past (like the flashbacks in "The Storm," "The Avatar and the Firelord," and "The Southern Raiders"). "Zuko Alone" and "Appa's Lost Days" have characters who don't or can't talk about their pasts quietly remembering them in the form of flashbacks.