The Collaborative International Dictionary of English v.0.48:

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 intercourse between
   persons related within the degrees wherein marriage is
   prohibited by law; broadly, sexual relations between closely
   related persons, usually taken as first cousins or closer.
   --Shak.
   [1913 Webster +PJC]

   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.
       [1913 Webster]