From IFWiki

A player is said to be stuck when he or she cannot think of a solution to a puzzle, or in a piece of puzzleless interactive fiction, he or she cannot think of anything interesting to do that will progress the story.

This is not the same as being in a deadlock, when there is no solution. If you're stuck, there is a solution, but you can't find it - at least not without looking harder.

See also