Final answer:
The term 'consanguinity' accurately fits the definition of a norm prohibiting sexual relations or marriage between certain relatives.
Step-by-step explanation:
The word that best fits the definition the norm forbidding sexual relations or marriage between certain relatives is consanguinity. Consanguinity is a concept that deals with the closeness of the blood relationship between individuals, oftentimes determining the legality and social acceptance of potential marriages within a society. Across cultures, marriage customs and practices, including polygamy, monogamy, endogamy, and exogamy, play a significant role in the organization of society and the formation of family units.