The term that fits the description of sexual activity between a married person and someone who is not their spouse is "adultery."
Explanation
Adultery is defined as voluntary sexual intercourse between a married person and someone who is not their spouse. This term is often used in legal, religious, and social contexts to describe a breach of marital fidelity. Adultery can have significant implications, including legal consequences in some jurisdictions, as well as social and personal ramifications for the individuals involved.