AKA The Reboot, AKA "BIG RED"
Any device that allows a writer to completely erase any already-occurred events of a story and bring the characters back to a predefined starting point, with little or no changes to them or their universe. Time travel ("It never happened"), parallel universes ("It never happened *here*"), unconscious duplicates ("We're all just clones/simulations/androids of the REAL characters!") and dream-sequences ("It was all a dream!") have all been used this way.
To be avoided unless the existence of such a phenomenon is, itself, the story's or series' central plot point (as in *The Man Who Folded Himself* or *The Left Hand of Darkness*). [suggested by Stephen J. Barringer]
Seen far too often in Star Trek. Way far too often. If something happens to your characters make it real.