epiphenomenon vs byproduct

epiphenomenon and byproduct both are nouns.

Word	Noun	Adjective	Verb	Adverb
epiphenomenon	Yes	No	No	No
byproduct	Yes	No	No	No

As nouns, byproduct is a hypernym of epiphenomenon; that is, byproduct is a word with a broader meaning than epiphenomenon:

epiphenomenon: a secondary phenomenon that is a by-product of another phenomenon
byproduct: a secondary and sometimes unexpected consequence

Other hypernyms of epiphenomenon include by-product.

epiphenomenon (noun)	byproduct (noun)
a secondary phenomenon that is a by-product of another phenomenon	a product made during the manufacture of something else
	a secondary and sometimes unexpected consequence