fly ball vs pop-up

fly ball and pop-up both are nouns.

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

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

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

fly ball (noun)	pop-up (noun)
(baseball) a hit that flies up in the air	a short high fly ball
	a store that is open for a limited period of time on temporary premises
	a book (usually for children) that contains one or more pages such that a three-dimensional structure rises up when a page is opened