Incest

n. sexual relations between individuals who are closely related

Incest is a sexual activity between two persons related by close kinship. In some societies this is prevented by the legal or social prohibition to marry. Although not universal, incest constitutes a cultural taboo in most current nations and many past societies, with legal penalties in some places. In some societies, like Ancient Egypt or Achaemenid Persia, brother–sister, father–daughter and mother–son relations were practiced.
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Incest /in·cest/ sexual activity between persons so closely related that marriage between them is legally or culturally prohibited. in·cest n. 1. [more]Incest - Community and Resources

Noun
1. sexual intercourse between persons too closely related to marry (as between a parent and a child)
(hypernym) unlawful carnal knowledge, criminal congress

n. incest, sexual relations between individuals who are closely related