The Collaborative International Dictionary of English v.0.48:

Fornication \For`ni*ca"tion\, n. [F. fornication, L.
   fornicatio.]
   1. Unlawful sexual intercourse on the part of an unmarried
      person; the act of such illicit sexual intercourse between
      a man and a woman as does not by law amount to adultery.
      [1913 Webster]

   Note: In England, the offense, though cognizable in the
         ecclesiastical courts, was not at common law subject to
         secular prosecution. In the United States it is
         indictable in some States at common law, in others only
         by statute. --Whartyon.
         [1913 Webster]

   2. (Script.)
      (a) Adultery.
      (b) Incest.
      (c) Idolatry.
          [1913 Webster]

WordNet (r) 3.0 (2006):

fornication
    n 1: voluntary sexual intercourse between persons not married to
         each other
    2: extramarital sex that willfully and maliciously interferes
       with marriage relations; "adultery is often cited as grounds
       for divorce" [syn: adultery, criminal conversation,
       fornication]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "fornication":
   act of love, adultery, aphrodisia, ass, balling, carnal knowledge,
   climax, cohabitation, coition, coitus, coitus interruptus,
   commerce, concubinage, congress, connection, copula, copulation,
   coupling, criminal conversation, cuckoldry, diddling,
   extracurricular sex, extramarital relations, free love,
   free-lovism, incest, intercourse, intimacy, lovemaking,
   making it with, marital relations, marriage act, mating, meat,
   onanism, orgasm, ovum, pareunia, premarital relations,
   premarital sex, procreation, relations, screwing, sex, sex act,
   sexual climax, sexual commerce, sexual congress,
   sexual intercourse, sexual relations, sexual union, sleeping with,
   sperm, venery



Easton's 1897 Bible Dictionary:

Fornication
   in every form of it was sternly condemned by the Mosaic law
   (Lev. 21:9; 19:29; Deut. 22:20, 21, 23-29; 23:18; Ex. 22:16).
   (See ADULTERY.)

     But this word is more frequently used in a symbolical than in
   its ordinary sense. It frequently means a forsaking of God or a
   following after idols (Isa. 1:2; Jer. 2:20; Ezek. 16; Hos. 1:2;
   2:1-5; Jer. 3:8,9).


Bouvier's Law Dictionary, Revised 6th Ed (1856):

FORNICATION, crim. law. The unlawful carnal knowledge of an unmarried person
with another, whether the latter be married or unmarried. When the party is
married, the offence, as to him or her, is known by the name of adultery.
(q.v.) Fornication is, however, included in every case of adultery, as a
larceny is included in robbery. 2 Hale's P. C. 302.