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-up, pop fly, 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