When the designer offers more than one conclusion to the end of the game. Some of these may be losing, winning, or optimal, but all describe some sort of resolution to the game's narrative.
An IF session can end in many unsatisfying ways -- the PC dies before the plot is resolved (see sudden death), the interpreter crashes, or the player simply loses interest and gives up. If a work abruptly ends the story whenever the player strays from a pre-determined plot, then it does not really have multiple endings, it is a rail game.
- Multiple Paths/Endings in the IF Gems review quotes collection (also available from the IF Archive).
- Multiple Endings at TV Tropes Wiki.