marriage vs polygamy

marriage and polygamy both are nouns.

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

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

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

marriage (noun)	polygamy (noun)
the act of marrying; the nuptial ceremony	the condition or practice of having more than one spouse at a time
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