Definition: Any ball that another ball is used to shoot at. The term derives from the fact that most of the time, an object ball is hit by the cue ball--the player is targeting on the ball as object.
Rarely, the cue ball itself is used as an object ball when it imparts motion to other balls that will set it in motion in return.
Examples:
The object ball is the last target to sight on for most pool strokes.
