Adultery is sexual infidelity
In many intimate relationships in many cultures there is usually an express or implied expectation of exclusivity, especially in sexual matters. Infidelity most commonly refers to a breach of the expectation of sexual exclusivity.Infidelity can occur in relation to physical intimacy and/or...

 to one's spouse, and is a form of extramarital sex
Extramarital sex
Extramarital sex occurs when a married person engages in sexual activity with someone other than his or her marriage partner.Where extramarital sexual relations breach a sexual norm it may also be referred to as adultery, fornication, philandery, or infidelity...

. It originally referred only to sex between a woman who was married and a person other than her spouse. Even in cases of separation from one's spouse, an extramarital affair is still considered adultery.

Adultery is illegal in some jurisdictions. The interaction between laws on adultery with those on rape
Rape is a type of sexual assault usually involving sexual intercourse, which is initiated by one or more persons against another person without that person's consent. The act may be carried out by physical force, coercion, abuse of authority or with a person who is incapable of valid consent. The...

 has and does pose particular problems in societies that are especially sensitive to sexual relations by a married woman and men.