extremum vs upper limit

extremum and upper limit both are nouns.

Word	Noun	Adjective	Verb	Adverb
extremum	Yes	No	No	No
upper limit	Yes	No	No	No

As nouns, upper limit is a hyponym of extremum; that is, upper limit is a word with a more specific, narrower meaning than extremum:

Other hyponyms of extremum include lower limit, minimum, maximum.

extremum (noun)	upper limit (noun)
the point located farthest from the middle of something	the limit on the upper (or northernmost) side of something
the most extreme possible amount or value	the largest possible quantity