epos vs heroic meter

epos and heroic meter both are nouns.

Word	Noun	Adjective	Verb	Adverb
epos	Yes	No	No	No
heroic meter	Yes	No	No	No

As nouns, heroic meter is a hyponym of epos; that is, heroic meter is a word with a more specific, narrower meaning than epos:

epos: a long narrative poem telling of a hero's deeds
heroic meter: a verse form suited to the treatment of heroic or elevated themes; dactylic hexameter or iambic pentameter

Other hyponyms of epos include chanson de geste, rhapsody, heroic, heroic verse.

epos (noun)	heroic meter (noun)
a long narrative poem telling of a hero's deeds	a verse form suited to the treatment of heroic or elevated themes; dactylic hexameter or iambic pentameter
a body of poetry that conveys the traditions of a society by treating some epic theme