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