Adultery is defined as voluntary sexual intercourse between a married person and someone other than their current spouse or partner. It is considered objectionable on social, religious, moral, or legal grounds. Historically, many cultures considered adultery a very serious crime, with penalties including capital punishment, mutilation, or torture. However, such punishments have gradually fallen into disfavor, especially in Western countries from the 19th century. In countries where adultery is still a criminal offense, punishments range from fines to caning and even capital punishment.

Adultery refers to sexual relations which are not officially legitimized. It does not refer to having sexual intercourse with multiple partners in the case of polygamy (when a man is married to more than one wife at a time, called polygyny; or when a woman is married to more than one husband at a time, called polyandry) .

In criminal law, adultery was a criminal offense in many countries in the past, and is still a crime in some countries today. However, since the introduction of the Matrimonial Causes Act in 1857, adultery has not been a crime in the United Kingdom. Adultery used to be one of the five grounds to cite a divorce, but this is no longer the case.

Adultery is regarded as a great social wrong and a great sin by God. Three types of adultery are spoken of in the Bible: sexual adultery, which is conjugal infidelity; adultery by a married man having sexual intercourse with a woman other than his wife; and adultery by a married woman having sexual intercourse with a man other than her husband.