One of three used in the game, generally a White, an Off-white and a Negative Cue Ball are used.

the ball that the billiard player or pool player strikes with his cue

the white (unnumbered) ball used to shoot at object (numbered) balls. cue ball in hand see Ball-in-hand.

the white ball used to knock coloured balls into the pockets

The white ball that is struck with the cue stick.

A usually white ball. In Most games the players start each shot by hitting their Pool cue into the cue ball in an attempt to score points or pocket balls.