extremum vs apex

extremum and apex both are nouns.

Word	Noun	Adjective	Verb	Adverb
extremum	Yes	No	No	No
apex	Yes	No	No	No

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

Other hyponyms of extremum include acme, peak, vertex.

extremum (noun)	apex (noun)
the point located farthest from the middle of something	the highest point (of something)
the most extreme possible amount or value	the point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars