Definitions:

1. In cue sports like billiards, pool, or snooke, is the ball that a player is aiming to hit or pocket with the cue ball. NOUN
   • He lined up his shot carefully, aiming the cue ball to strike the object ball into the corner pocket.
2. The billiard ball that is intended to be the first ball struck by the cue ball. NOUN
   • Missing the object ball completely is called a foul.

Synonyms:

• legal ball
• scoring ball
• target ball