pool ball vs eight ball

pool ball and eight ball both are nouns.

Word	Noun	Adjective	Verb	Adverb
pool ball	Yes	No	No	No
eight ball	Yes	No	No	No

As nouns, eight ball is a hyponym of pool ball; that is, eight ball is a word with a more specific, narrower meaning than pool ball:

pool ball: ball used in playing pool
eight ball: a black pool ball bearing the number 8; should be the last to go in certain pool games

pool ball (noun)	eight ball (noun)
ball used in playing pool	a black pool ball bearing the number 8; should be the last to go in certain pool games