Simple Definition:

Polygamy is the practice of having more than one spouse at the same time. [Spouse refers to a husband or a wife.] This can involve either multiple husbands or multiple wives.

For example, a man who is married to two women at the same time is practicing polygamy. Similarly, a woman who is married to two men at the same time is also practicing polygamy.

Very Simple Definition:

Polygamy is when a person is married to more than one person at the same time. For example, a man who has two wives is practicing polygamy.