incest

n. Sexual interaction between close blood relatives that is prohibited by a culture and may or may not also be illegal. The definition of incest is variable among cultures and among groups within cultures so that parent-child or sibling sexual relations are almost always banned but were allowed among, for instance, Egyptian royalty in some periods of history in order to preserve and concentrate the royal bloodline. Sexual activity among other close relatives such as first cousins, aunts, uncles, and grandparents is also usually forbidden within a culture, but in some cultures adolescents are introduced to sex by relatives of the parental or grandparental generation, and some sex acts, such as swallowing semen, are considered not sexual acts but a transmission of power from one generation to the next.