Polygyny is a
marital practice in which a man has more than one wife simultaneously.
This is the usual form of
polygamy.
The man may marry more than one woman at the same time, or marry one or more other women while he is already married.
The opposite form—
polyandry—where a woman has more than one husband simultaneously, is much less common.
The term "polygyny" is also used to refer more generally to a male breeding with more than one female.
In social insects this refers to colonies having multiple queens.