upper limit vs extremum

upper limit and extremum both are nouns.

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

As nouns, extremum is a hypernym of upper limit; that is, extremum is a word with a broader meaning than upper limit:

Other hypernyms of upper limit include peak, large indefinite amount, large indefinite quantity.

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