1. the situation in which one partner in a marriage or intimate relationship becomes sexually or emotionally involved with a person other than the partner’s spouse or girlfriend or boyfriend. The infidelity, also called cheating, usually transpires in secrecy between those in the extradyadic relationship; however, online infidelity, in which the extradyadic intimacy occurs on social networks and other Internet-based sites that may be only marginally private, is becoming widespread. Regardless of its method of pursuit, its effects can be harmful: Infidelity has been linked to significant adverse emotional and behavioral consequences, especially for the individual who is cheated on, including decreased self-esteem and increased risk of depression and suicide. Additionally, it is cited as a common cause of divorce or relationship dissolution. See also adultery.

2. more generally, any disloyalty or lack of faith.