Adultery is referred to as extramarital sex, philandery, 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.

Adultery is illegal in some countries. The interaction between laws on adultery with those on rape has and does pose particular problems in societies that are especially sensitive to sexual relations by a married woman and men, such as some Muslim countries. The difference between the offenses is that adultery is voluntary, while rape is not.

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. Hinduism has a similar concept.

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, where there is fault-based family law, it almost always constitutes 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.

Three recent studies in the United States, using nationally representative samples, have found that about 10–15% of women and 20–25% of men admitted to having engaged in extramarital sex.

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