Polygamy
Polygyny
noun, の adjective
One husband, many wives. This is polygamy.
More specifically, 一夫多妻 is a term for polygyny, or marriage between one man and multiple women. You'll mostly run into it in period dramas or discussions of history and anthropology, since it was common among the ruling elite of certain societies during premodern times.
This is a jukugo word, which means it will probably use on'yomi readings for each kanji. If you can remember those, you might be able to read this word! Just note that 一 changes from いち to いっ, and 夫 changes from ふう to ぷ for some reason.
You can remember this because you think people who practice polygamy should eat poo (いっぷ). You think guys with multiple wives are gross misogynists, so whenever you meet one you say, "Hey, polygamist — eat poo!"
たとえ一夫多妻がOKになっても、お金がなければ少子化は止められないでしょ。
Even if polygamy were to be accepted, without financial stability, it's unlikely that we could halt the declining birthrate, right?
え、日本で一夫多妻をしてる人達もいるんですか！？
Oh, are there people in Japan who practice polygamy?
チョウチンアンコウやモリアオガエル、ミツバチなんかは一夫多妻の反対で、一妻多夫なんですよ。
Chimera anglerfish, Japanese tree frogs, and honey bees, for example, have a mating system opposite to polygyny, called polyandry.