(a) In billiards: the red ball which is placed on the mark (but it is doubtful whether this sense has ever been practically in English use). (b)…

A red ball in snooker, billiards, and similar games.

Sport and Games. With the. Either of the white balls in billiards; (also) the white ball in pool or snooker.

In various elliptical uses. In Billiards, Snooker, and similar games: a black ball.

In snooker and similar games: a green ball; (also) an attempt to pot a green ball.