Monogamy refers to the state of being legally married to only one spouse at a time. Appearing in two general forms, monogamy may imply a lifelong contract between two individuals that may be broken only under penalty.
It is used in opposition to bigamy (the condition of having two wives or two husbands at the same time) and polygamy (offense of willfully and knowingly having more than one wife or husband at the same time).
An excerpt from Daniel Engber's article Are Humans Monogamous or Polygamous? says:
According to anthropologists, only 1 in 6 societies enforces monogamy as a rule.
In The Myth of Monogamy, evolutionary psychologists David P. Barash and Judith Eve Lipton say we're not the only pair-bonding species that likes to sleep around. Even among the animals that have long been known as faithful types—nesting birds, etc.—not too many stay exclusive. Most dally. "There are a few species that are monogamous," says Barash. "The fat-tailed dwarf lemur. The Malagasy giant jumping rat. You've got to look in the nooks and crannies to find them, though." Like so many other animals, human beings aren't really that monogamous.