Polygamy is a form of polyamory where a person has more than one spouse at one time. It is also called plural marriage or bigamy (though strictly, bigamy is having just two spouses). In the United Kingdom, this is illegal.
Polygamy includes both polygyny and polyandry as well as potentially including the rare circumstances where more than one woman is married to more than one man.
Polygamy as an institution continues in much of the developing world. However, even in cultures that permit polygamy, its practice may nevertheless be discouraged. The Islamic Qu'ran, for example, suggests men restrict themselves to one wife: "If you have more than one wife, you will never be able to treat them equitably...and if you cannot treat them equitably then you should not engage in the practice at all."
See Polyamory for the main article on this area.