Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

incest
     n : sexual intercourse between persons too closely related to
         marry (as between a parent and a child)