Which term refers to sexual relations between closely related individuals?

The term that refers to sexual relations between closely related individuals is indeed incest. It specifically denotes sexual activity between family members who are typically prohibited from such relationships due to legal, cultural, or moral reasons. This concept is important in various legal systems and social contexts, as it raises both ethical concerns and implications for biological offspring, such as an increased risk of genetic disorders.

Adultery refers to a married person engaging in sexual relations outside of their marriage, which is not necessarily related to familial ties. Infidelity generally encompasses any breach of commitment in a romantic relationship, mainly relating to emotional and physical fidelity, again without the familial aspect. Forbidden relations is a broader and more vague term that could apply to any relationship considered taboo, but it doesn't specifically target the context of sexual relations between closely related individuals. Therefore, incest is the precise term that accurately identifies the specific nature of the relationships described in the question.