polygamy vs marriage

polygamy and marriage both are nouns.

Word	Noun	Adjective	Verb	Adverb
polygamy	Yes	No	No	No
marriage	Yes	No	No	No

As nouns, marriage is a hypernym of polygamy; that is, marriage is a word with a broader meaning than polygamy:

polygamy: the condition or practice of having more than one spouse at a time
marriage: the state of being a married couple voluntarily joined for life (or until divorce)

Other hypernyms of polygamy include matrimony, spousal relationship, union, wedlock.

polygamy (noun)	marriage (noun)
the condition or practice of having more than one spouse at a time	the act of marrying; the nuptial ceremony
	two people who are married to each other
	the state of being a married couple voluntarily joined for life (or until divorce)
	a close and intimate union