Adultery, or marital infidelity is generally defined as consensual sexual intercourse by a married person with someone other than their lawful spouse.
As there is usually an implicit or explicit agreement between spouses to not have sex outside the marriage, the common synonym for adultery is infidelity as well as unfaithfulness or in colloquial speech, cheating.
The sexual partner of a person committing adultery or infidelity is often referred to in legal documents (especially divorce proceedings) as a co-respondent, while the person whose spouse has been unfaithful is often labeled a cuckold; originally, the latter term was applied only to males, but in more recent times women have been characterized in this way too.
Different couples (or groups)—and even different individuals—may have different ideas of what constitutes infidelity. A person may not want his or her partner flirting with anyone else, may accept that but draw the line at petting. A person who is identifies as heterosexual may accept his or her partner engaging in homosexual but not heterosexual acts with others, or vice versa.
A marriage in which both spouses agree that it is acceptable to have sexual relationships with other people is termed open marriage and the resulting sexual relationships are generally not considered marital infidelity, at least from a non-legal standpoint.
The law in some areas may not recognize open-marriage agreements and thus such extramarital sex may be considered adultery nonetheless. Sometimes only one party in an open marriage will opt to have other sexual relationships, in which case the one who does not do so is referred to as a wittol: sometimes called a "contented cuckold".
