Adultery is a nearly universal concern. It is defined by Judaic, Christian, and Muslim formulations, and in legal codes deriving from Roman law. Known colloquially as cheating or infidelity, adultery is more complex than simple faithlessness and is not to be confused with fornication, or sex between two unmarried people. In its simplest definition, adultery occurs when a married person has sex with someone other than his or her spouse. Marriage is requisite, on the one hand, for an action to be called adultery. On the other hand, the cheating couple also must not be married to one another; otherwise the situation is not adultery but bigamy or polygamy, even in jurisdictions where such is proscribed. It is sometimes the case that spouses agree beforehand that one or both spouses will seek sexual pleasure outside their marriages, a situation that has been called open marriage. Regardless of the level of consent or of participation by the offended spouse, most legal and religious authorities still consider such activity to be adulterous.

Simple definitions aside, whether a particular act can be called adultery depends very much on historical, legal, and cultural contexts. That is, what qualifies as adultery in one jurisdiction would not in another, or even in the same jurisdiction in another era. In some definitions, both ''cheating'' partners are adulterers if either of them is married, and each is to be treated similarly. In practice, one partner--usually the woman--is often punished more severely than the other. In many definitions, a married man commits adultery only if he has sex with a married woman not his wife; if the man's paramour is not married, neither is an adulterer. Under the same definitions, a married woman commits adultery when she has sex outside her marriage, regardless of the marital status of her partner. The primary variable in this diversity