Incest \In"cest\, n. [F. inceste, L. incestum unchastity, incest, fr. incestus unchaste; pref. in- not + castus chaste. See Chaste.]

The crime of cohabitation or sexual commerce between persons related within the degrees wherein marriage is prohibited by law. --Shak.

Spiritual incest. (Eccl. Law)
   (a) The crime of cohabitation committed between persons who have a spiritual alliance by means of baptism or confirmation.
   (b) The act of a vicar, or other beneficiary, who holds two benefices, the one depending on the collation of the other.

Source: Webster's Revised Unabridged Dictionary (1913)