A term used in the board game go. A ladder breaker is a stone played at a location which would allow a stone captured in shicho (or ladder) to escape.
The ability to read out shichos in advance to look for ladder breakers, and the ability to be aware of the existence of ladder breakers is an important part of learning to play go well.
If the player that has a key stone caught in a ladder is able to play a ladder breaker without his opponent noticing, it will have a large effect on the outcome of the game.