What is the term used for sexual intercourse between blood relatives?

The term used for sexual intercourse between blood relatives is indeed incest. Incest is defined as sexual relations between individuals who are closely related by blood, such as siblings, parents and children, or other close family members. This practice is not only socially frowned upon in many cultures but is also illegal in various jurisdictions due to the potential for negative genetic implications in offspring and the violation of social norms regarding familial relationships.

Infidelity refers to being unfaithful in a committed relationship, typically implying sexual relations with someone outside of that relationship, and is not specifically about blood relations. Adultery, similar to infidelity, involves a married person engaging in sexual relations outside of their marriage and does not inherently involve blood relatives. Polygamy involves being married to more than one person simultaneously and also does not pertain to the relationships of blood relatives.

Thus, the correct answer accurately identifies the specific nature of the relationship defined in the question.