axiom vs apophthegm

axiom and apophthegm both are nouns.

Word	Noun	Adjective	Verb	Adverb
axiom	Yes	No	No	No
apophthegm	Yes	No	No	No

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

Other hyponyms of axiom include aphorism, apothegm, gnome, moralism.

axiom (noun)	apophthegm (noun)
a saying that is widely accepted on its own merits	a short pithy instructive saying
(logic) a proposition that is not susceptible of proof or disproof; its truth is assumed to be self-evident