The Collaborative International Dictionary of English v.0.48:

Adultery \A*dul"ter*y\, n.; pl. Adulteries. [L. adulterium.
   See Advoutry.]
   1. The unfaithfulness of a married person to the marriage
      bed; sexual intercourse by a married man with another than
      his wife, or voluntary sexual intercourse by a married
      woman with another than her husband.
      [1913 Webster]

   Note: It is adultery on the part of the married wrongdoer.
         The word has also been used to characterize the act of
         an unmarried participator, the other being married. In
         the United States the definition varies with the local
         statutes. Unlawful intercourse between two married
         persons is sometimes called double adultery; between
         a married and an unmarried person, single adultery.
         [1913 Webster]

   2. Adulteration; corruption. [Obs.] --B. Jonson.
      [1913 Webster]

   3. (Script.)
      (a) Lewdness or unchastity of thought as well as act, as
          forbidden by the seventh commandment.
      (b) Faithlessness in religion. --Jer. iii. 9.
          [1913 Webster]

   4. (Old Law) The fine and penalty imposed for the offense of
      adultery.
      [1913 Webster]

   5. (Eccl.) The intrusion of a person into a bishopric during
      the life of the bishop.
      [1913 Webster]

   6. Injury; degradation; ruin. [Obs.]
      [1913 Webster]

            You might wrest the caduceus out of my hand to the
            adultery and spoil of nature.         --B. Jonson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adultery
    n 1: 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:

71 Moby Thesaurus words for "adultery":
   act of love, adulterous affair, affair, amour, aphrodisia, ass,
   balling, carnal knowledge, climax, cohabitation, coition, coitus,
   coitus interruptus, commerce, concubinage, congress, connection,
   copula, copulation, coupling, criminal conversation, cuckoldry,
   diddling, entanglement, eternal triangle, extracurricular sex,
   extramarital relations, flirtation, forbidden love, fornication,
   free love, free-lovism, hanky-panky, illicit love, incest,
   infidelity, intercourse, intimacy, intrigue, liaison, love affair,
   lovemaking, making it with, marital relations, marriage act,
   mating, meat, onanism, orgasm, ovum, pareunia,
   premarital relations, premarital sex, procreation, relations,
   romance, romantic tie, screwing, sex, sex act, sexual climax,
   sexual commerce, sexual congress, sexual intercourse,
   sexual relations, sexual union, sleeping with, sperm, triangle,
   unfaithfulness, venery



Easton's 1897 Bible Dictionary:

Adultery
   conjugal infidelity. An adulterer was a man who had illicit
   intercourse with a married or a betrothed woman, and such a
   woman was an adulteress. Intercourse between a married man and
   an unmarried woman was fornication. Adultery was regarded as a
   great social wrong, as well as a great sin.

     The Mosaic law (Num. 5:11-31) prescribed that the suspected
   wife should be tried by the ordeal of the "water of jealousy."
   There is, however, no recorded instance of the application of
   this law. In subsequent times the Rabbis made various
   regulations with the view of discovering the guilty party, and
   of bringing about a divorce. It has been inferred from John
   8:1-11 that this sin became very common during the age preceding
   the destruction of Jerusalem.

     Idolatry, covetousness, and apostasy are spoken of as adultery
   spiritually (Jer. 3:6, 8, 9; Ezek. 16:32; Hos. 1:2:3; Rev.
   2:22). An apostate church is an adulteress (Isa. 1:21; Ezek.
   23:4, 7, 37), and the Jews are styled "an adulterous generation"
   (Matt. 12:39). (Comp. Rev. 12.)


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

ADULTERY, criminal law. From ad and alter, another person; a criminal
conversation, between a man married to another woman, and a woman married to
another man, or a married and unmarried person. The married person is guilty
of adultery, the unmarried of fornication. (q.v.) 1 Yeates, 6; 2 Dall. 124;
but see 2 Blackf. 318.
     2. The elements of this crime are, 1st, that there shall be an unlawful
carnal connexion; 2dly, that the guilty party shall at the time be married;
3dly, that he or she shall willingly commit the offence; for a woman who has
been ravished against her will is not guilty of adultery. Domat, Supp. du
Droit Public, liv. 3, t. 10, n. 13.
     3. The punishment of adultery, in the United States, generally, is fine
and imprisonment.
     4. In England it is left to the feeble hands of the ecclesiastical
courts to punish this offence.
     5. Adultery in one of the married persons is good cause for obtaining a
divorce by the innocent partner. See 1 Pick. 136; 8 Pick. 433; 9 Mass. 492:
14 Pick. 518; 7 Greenl. 57; 8 Greenl. 75; 7 Conn. 267 10 Conn. 372; 6 Verm.
311; 2 Fairf. 391 4 S. & R. 449; 5 Rand. 634; 6 Rand. 627; 8 S. & R. 159; 2
Yeates, 278, 466; 4 N. H. Rep. 501; 5 Day, 149; 2 N. & M. 167.
     6. As to proof of adultery, see 2 Greenl. Sec. 40, Marriage.