1. A rack in the form of an equilateral triangle. There are different sizes of triangles for racking different games (which use different ball sizes and numbers of balls), including the fifteen ball racks for snooker and various pool games such as eight-ball and blackball. A larger triangle is used for the twenty-one ball rack for baseball pocketbilliards). The smallest trianglerack is employed in three-ball (see illustration at that article) but is not strictly necessary, as the front of a larger rack can be used, or the balls can be arranged by hand.
Further information: Rack (billiards)
2. The object balls in triangular formation, before the breakshot, after being racked as above (i.e., same as rack, definition 2). Principally British. (See also pyramid.)