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)

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "flashback":
   commitment to memory, exercise of memory, hindsight,
   learning by heart, looking back, memoir, memorization, memorizing,
   recall, recalling, recollecting, recollection, reconsideration,
   reflection, remembering, remembrance, reminiscence, retrospect,
   retrospection, review, rote, rote memory, study