flashback

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A literary or cinematic device in which an earlier event is inserted into the normal chronological order of a narrative.
• noun The episode or scene depicted by means of this device.
• noun Psychiatry A recurring, intensely vivid mental image of a past traumatic experience.
• noun An unexpected recurrence of the effects of a hallucinogenic drug long after its original use.
• noun A vivid memory that arises spontaneously or is provoked by an experience.
• noun An experience that has characteristics of an earlier experience.

from the GNU version of the Collaborative International Dictionary of English.

• noun a transition (in literary or theatrical works or films) to an earlier event or scene; also, the scene thus introduced.
• noun An unusually vivid recollection of a prior experience, often one that is traumatic, such as scenes from combat or a criminal assault, or induced by hallucinogenic drugs such as LSD; when accompanied by hallucinations it is called flashback hallucinosis.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a dramatic device in which an earlier event is inserted into the normal chronological flow of a narrative
• noun psychology a vivid mental image of a past trauma, especially one that recurs
• noun a similar recurrence of the effects of a hallucinogenic drug

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb return in time
• noun an unexpected but vivid recurrence of a past experience (especially a recurrence of the effects of an hallucinogenic drug taken much earlier)
• noun a transition (in literary or theatrical works or films) to an earlier event or scene that interrupts the normal chronological development of the story