(noun)
1. A geometric form, usually aluminum, wooden or plastic, used to assist in setting up balls in games like eight-ball , nine-ball , and snooker . The rack allows for more consistently tight grouping of balls, which is necessary for a successful break shot . In most games a triangle -shaped rack capable of holding fifteen balls can be employed, even if the game calls for racking less than a full ball set , such as in the game of nine-ball . For further information, see the Rack (billiards) main article.
2. Used to refer to a racked group of balls before they have been broken.
3. In some games, refers to a single frame .
4. Colloquial shorthand for "a set of balls".
5. Short for cue rack , wall rack or scoring rack when such abbreviation would not be ambiguous.

(verb)
The act of setting up the balls for a break shot . In tournament play this will be done by the referee , but in lower-level play, players either rack for themselves or for each other depending on convention.