fly ball vs pop fly

fly ball and pop fly both are nouns.

Word	Noun	Adjective	Verb	Adverb
fly ball	Yes	No	No	No
pop fly	Yes	No	No	No

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

Other hyponyms of fly ball include blast, pop-fly, pop-up, line drive, liner, flare, Texas leaguer.

fly ball (noun)	pop fly (noun)
(baseball) a hit that flies up in the air	a short high fly ball