flashback (flash|back)

noun

• 1a scene in a film, novel, etc. set in a time earlier than the main story: in a series of flashbacks, we follow the pair through their teenage years [mass noun]: the movie tells the story in flashback
• a disturbing sudden vivid memory of an event in the past, typically as the result of psychological trauma or taking LSD.

• 2a flame moving rapidly back through a combustible vapour: cooling the area prevented a flashback