A trebuchet is a type of reciprocal zugzwang occuring in pawn endgames, where whoever it is to move loses. The position below is the most common trebuchet position.
In the diagram below, White is to play, and wants to reach the above trebuchet position with Black to move, as we have seen.
Below is rarer example of a trebuchet. If black is to move, then he only has one legal move, 1...b3, which allows white to win by 2 axb3#. With white to move black wins easily.