In
cinema: a flashback is a
scene (or shot) which portrays events in the
diegetic past relative to the scene's (or shot's) placement, linearly, in the
film.
Translation: Assume scenes A, B, and C where scene A is shown first, scene B is shown second, and scene C is shown third. The linear progression of the scenes, then, is A, B, C. If scene B depicts events which occur, diegetically, before the events depicted in scene A, then scene B can be considered a flashback relative to scene A.
Further translation: If the stuff you're seeing now happens before the stuff you've already seen, it's probably a flashback.
It can become even more complicated. If scene C, above, "happens" before scene B, what might that be? Is it a flashback within a flashback? Or is it just another flashback? Consider a film like Memento, in which chronological story information is revealed "backward" relative to the progression of the scenes. Is each scene a flashback? Or are you simply watching a story being told backward? Or both?
This sort of debate is common when defining film terminology. Ask anyone for a precise definition of a "close up" or a "medium shot", for example, and you might be in for either a lecture or an argument.