Adultery is the voluntary sexual intercourse between a married person and another person who is not his or her spouse, though in many places it is only considered adultery when a married woman has sexual relations with someone who is not her husband. In most cases, in western countries, only the married party is said to have committed adultery, and if both parties are married (but not to each other) then they both commit separate acts of adultery. In other countries, both parties to the adultery are considered guilty, while in others again only the woman is able to commit adultery and to be considered guilty.

Adultery is also referred to as extramarital sex, philandary or infidelity but does not include fornication. The term "adultery" for many people carries a moral or religious association, while the term "extramarital sex" is morally or judgmentally neutral.

The interaction between laws on adultery with those on rape has and does pose particular problems in societies which are especially sensitive to sexual relations by a married woman, such as some Muslim countries. The difference between the offenses is that adultery is voluntary, while rape is not. If a woman claims that she has been raped, and the offense cannot be proved, then a conclusion that the sexual relations were voluntary may be drawn, and the consequences of adultery may result, including an honour killing. In those circumstances, a woman victim would be reluctant to report a rape against her.

The term adultery has a Judeo-Christian origin, though the concept of marital fidelity predates Judaism and is found in many other societies. Though the definition and consequences vary between religions, cultures and legal jurisdictions, the concept is similar in Judaism, Christianity and Islam, and Hinduism has a similar concept. But the word should be used cautiously when discussing various cultures, some of which permit less permanent forms of marriage, or even sexual "lending".

Historically, adultery has been considered to be a serious offense by many cultures. In some countries, adultery is a crime. However, even in jurisdictions where adultery is not itself a criminal offense, it may still have legal consequences, particularly in divorce cases. For example it may constitute grounds for divorce, it may be a factor to consider in a property settlement, it may affect the status of children, the custody of children, etc. Moreover adultery can result in social ostracism in some parts of the world.

Adultery is quite common. Three recent studies in the United States, using nationally representative samples, have found that about 10-15% of women and 20-25% of men had engaged in extramarital sex.Clements, M. (1994, August 7). Sex in America today: A new national survey reveals how our attitudes are changing. Parade Magazine, 4-6.Laumann, E. O., Gagnon, J. H., Michael, R. T, & Michaels, S. (1994). The social organization of sexuality: Sexual practices in the United States. Chicago: University of Chicago Press.