Adultery (from Latin adulterium) is extramarital sex that is considered objectionable on social, religious, moral, or legal grounds. Although the sexual activities that constitute adultery vary, as well as the social, religious, and legal consequences, the concept exists in many cultures and is similar in Christianity, Judaism and Islam. Adultery is viewed by many jurisdictions as offensive to public morals, being a mistreatment of the marriage relationship.

Historically, many cultures considered adultery a very serious crime, some subject to severe punishment, usually for the woman and sometimes for the man, with penalties including capital punishment, mutilation, or torture. 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. Since the 20th century, criminal laws against adultery have become controversial, with most Western countries decriminalizing adultery.