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]

WordNet (r) 3.0 (2006):

incest
    n 1: sexual intercourse between persons too closely related to
         marry (as between a parent and a child)

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "incest":
   active algolagnia, adultery, algolagnia, algolagny, amphierotism,
   autoeroticism, bisexuality, concubinage, coprophilia,
   criminal conversation, cuckoldry, exhibitionism,
   extracurricular sex, extramarital relations, fetishism,
   fornication, free love, free-lovism, heterosexuality,
   homoeroticism, homosexualism, homosexuality, incestuousness,
   lesbianism, masochism, narcissism, necrophilia, paraphilia,
   passive algolagnia, pedophilia, premarital relations,
   premarital sex, sadism, sadomasochism, sapphism, scotophilia,
   sexual inversion, sexual normality, sexual preference,
   swinging both ways, transvestitism, tribadism, tribady, voyeurism,
   zooerastia, zoophilia