From IFWiki

A game state from which the player can still reach the optimal ending (or, for a multipath game, any set of endings that count as "wins"). If the player has performed an action that irreversibly closes off the winning ending(s), or if the game is too buggy to be winnable from a starting position, the game is said to be unwinnable.

In some games, the player can check the status of a current session by typing the command WINNABLE.