flashback has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is flash-forward.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a transition (in literary or theatrical works or films) to an earlier event or scene that interrupts the normal chronological development of the story
an unexpected but vivid recurrence of a past experience (especially a recurrence of the effects of an hallucinogenic drug taken much earlier)
Synonyms, antonyms and the broader and narrower relations for flashback come from Princeton University’s WordNet. Full sources and licences.