Flashback

A flashback is a plot device used to show the past of a character. When done right, it is subtle and wouldn't even necessarily be called a flashback; they can be hidden in the form of dreams, for one. However, when done wrong (which is very common, especially in fanfiction when a character's past needs divulging) they can seem trite and almost offensively obvious.

Use in Fiction
Many authors successfully use flashbacks, though they are altogether more common in movies and the like. Some authors will simply change to present-tense or italics to indicate that a flashback is taking place; others introduce it through a dream or a meditative state. Amateur authors will sometimes put notices such as "FLASHBACK:" just before typing, which can appear crude to the reader. The best advice is to either use a subtle, noninvasive flashback or to avoid writing one at all.

Resources

 * Original Writerium Article