What is defined as a sexual act between a married person and someone other than their spouse?

The term that describes a sexual act between a married person and someone other than their spouse is adultery. This concept is recognized in various religions and cultures as a violation of the marital vows that typically emphasize loyalty and faithfulness. Adultery often carries significant moral, ethical, and sometimes legal implications, as it can lead to the breakdown of trust within a marriage and affect family dynamics.

Fidelity refers to faithfulness to a spouse, which is the opposite of what is described in the question. Cohabitation involves two individuals living together in a romantic relationship without being married, which does not specifically relate to the act defined. Promiscuity refers to having multiple sexual partners without commitment, which may involve married individuals but is a broader term that lacks the specific focus on the marital context indicated in the question. Thus, adultery is the precise term that defines the scenario presented.