Depending on context:
1. Any ball that may be legally struck by the cue ball (i.e., any ball-on);
2. Any ball other than the cue ball .
Usage notes: When speaking very generally, e.g. about the proper way to make a kind of shot , any ball other than the cue ball is an object ball . In narrower contexts, this may not be the case. For example when playing eight-ball one might not think of the 8 ball as an object ball unless shooting for the 8.