Monogamy

Monogamy (; "monos+gamos" meaning "one+marriage") is a form of marriage in which a person or animal has only one spouse at any one time.
Etymology.
The word "monogamy" comes from the Greek words "μονός", "monos" which means one or alone, and "γάμος", "gamos" which means marriage.
Religion.
Monogamy is one of the beliefs of Judaism and Christianity (except for the fundamentalist branch of The Church of Jesus Christ of Latter-day Saints (Mormons) who strictly follow the teachings of Joseph Smith, who allowed polygamy and had many wives). In Jewish and Christian doctrine, marriage is between one husband and wife.
Law.
In most countries, monogamy is the only legal form of marriage, either between a man and a woman in most of these countries, and between two persons of the same gender (gay marriage) mainly in the Western world. 
In these countries polygamy is illegal, and a person who has more than one spouse can be prosecuted for bigamy.