Polygamy describes the custom of being married to more than one spouse. In polyamorous terms, to be polygamous can be defined as being married to more than one person OR to have many sexual partners within a short interval of one another.
Polygamy is traditionally based around a male having multiple wives. But technically the term can refer to any gender having multiple spouses consisting of members of any other gender.
The opposite of polygamy is monogamy.
Brett and his wife were open to the idea of polygamy. After some careful consideration, they decided that polygamy wouldn’t be a bad idea. Brett now has two wives.