A player is in deadlock when he or she is facing a puzzle to which no solution exists.
A deadlock can arise for example if the player destroys an object that is needed to solve a puzzle, or somehow manages to drop the key to a locked door on the wrong side of a door.
Deadlocks are usually considered as game-level bugs - the IF author should attempt to prevent deadlocks from happening, or at least detect them and handle the situation.
There is a (perhaps overly) fine distinction between deadlocks and unwinnable games. In an unwinnable game, there is no way to reach the final winning ending. This is a kind of deadlock, and in a game where there is only one way to reach the single winning ending, any deadlock makes the game unwinnable.
However, in a "sprawling" game, or games with multiple endings, it is possible for a single puzzle to become an unsolvable deadlock, but there may be other ways of progressing the overall game, by working around the puzzle rather than solving it. The problem arises when the player is bent on solving the puzzle at hand, when in fact he or she should zoom out and try to find another way to do things.