polygamy vs polygyny

polygamy and polygyny both are nouns.

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

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

polygamy: the condition or practice of having more than one spouse at a time
polygyny: a form of polygamy in which a man has multiple wives

Other hyponyms of polygamy include polyandry.

polygamy (noun)	polygyny (noun)
the condition or practice of having more than one spouse at a time	a form of polygamy in which a man has multiple wives