Final answer:
The word that best fits the definition of the norm forbidding sexual relations between certain relatives is incest. Incest is a universal taboo that distinguishes between familial relations and potential sexual partners.
Step-by-step explanation:
The word from the list that best fits the definition of the norm forbidding sexual relations or marriage between certain relatives is incest. The taboo against incest is universally present in societies around the world and often includes prohibitions against sexual relations between parents and their children, as well as potentially between other close relatives. This societal norm helps define the boundaries between family relationships and those relationships where sexual interaction is appropriate.
It is important to differentiate between incest and other marital structures such as polygamy, which is the marriage of more than two adults, polygyny, where one man has multiple wives, and monogamy, where a marriage exists between only two individuals. Unlike bigamy, where an individual is married illegally to more than one person at a time, incest specifically refers to the prohibition against sexual relations with certain relatives, which is a norm shared across cultures regardless of their marriage customs.