object ball vs billiard ball

object ball and billiard ball both are nouns.

Word	Noun	Adjective	Verb	Adverb
object ball	Yes	No	No	No
billiard ball	Yes	No	No	No

As nouns, billiard ball is a hypernym of object ball; that is, billiard ball is a word with a broader meaning than object ball:

object ball: the billiard ball that is intended to be the first ball struck by the cue ball
billiard ball: ball used in playing billiards

object ball (noun)	billiard ball (noun)
the billiard ball that is intended to be the first ball struck by the cue ball	ball used in playing billiards