The Collaborative International Dictionary of English v.0.48:

flashback \flashback\ n.
   1. a transition (in literary or theatrical works or films) to
      an earlier event or scene; also, the scene thus
      introduced.
      [WordNet 1.5]

   2. 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.
      [PJC]

WordNet (r) 3.0 (2006):

flashback
    n 1: a transition (in literary or theatrical works or films) to
         an earlier event or scene that interrupts the normal
         chronological development of the story [ant: flash-
         forward]
    2: an unexpected but vivid recurrence of a past experience
       (especially a recurrence of the effects of an hallucinogenic
       drug taken much earlier)