As in film, a flashback in literature is a technique which takes the narrative back in time from the point the story has reached, to recount events that happened before and give the "back-story." It was used extensively by author Ford Madox Ford.
A flashback can also occur in the mind, either voluntarily remembering a past experience, or involuntarily due to a disorder such as posttraumatic stress disorder (PTSD), or a hallucinogen such as LSD. Flashbacks are often reported years after a subject was exposed to the drug or trauma.
Flashback is also the name of a (highly) controversial Swedish media group (flashback.se, flashback.net, flashback.org) run by Jan Axelsson .
Flashback: The Quest for Identity is a computer game.