A term used in almost all pool and billiards games used to describe the first shot.
In games like Eight Ball and Nine Ball this shot is usually made with great force so that the maximum amount of dispersion of object balls occurs. The intent is to pocket a ball without scratching.
In games like Straight Pool, One Pocket, and Snooker the object of the break is usually to play a safety so that your opponent doesn't have an open shot and is forced to leave you a shot.
In Three Cushion Billiards the object on the break is to hit the red object ball and have the cue ball hit three rails before hitting your opponents cue ball.