During a game of pool (pocket billiards) a combination shot is one in which the ball to be pocketed (the called ball) is different from the object ball.
The most common form of combination shot is a two-way combo - in which the player causes the cue ball to strike the object ball in such a way that it, in turn, pockets the called ball. Three-way combos are also seen occasionally during a game of pool.
The game of nine ball is especially conducive to the use of combination shots, due to the fact that a player usually doesn't have to call a shot. This leads to wild attempts to 'luck in' a shot - often by attempting a combo.