The Collaborative International Dictionary of English v.0.48:

Bigamy \Big"a*my\, n. [OE. bigamie, fr. L. bigamus twice
   married; bis twice + Gr. ? marriage; prob. akin to Skt.
   j[=a]mis related, and L. gemini twins, the root meaning to
   bind, join: cf. F. bigamie. Cf. Digamy.] (Law)
   The offense of marrying one person when already legally
   married to another. --Wharton.
   [1913 Webster]

   Note: It is not strictly correct to call this offense bigamy:
         it more properly denominated polygamy, i. e., having a
         plurality of wives or husbands at once, and in several
         statutes in the United States the offense is classed
         under the head of polygamy.
         In the canon law bigamy was the marrying of two virgins
         successively, or one after the death of the other, or
         once marrying a widow. This disqualified a man for
         orders, and for holding ecclesiastical offices.
         Shakespeare uses the word in the latter sense.
         --Blackstone. --Bouvier.
         [1913 Webster]

               Base declension and loathed bigamy. --Shak.
         [1913 Webster]

WordNet (r) 3.0 (2006):

bigamy
    n 1: having two spouses at the same time
    2: the offense of marrying someone while you have a living
       spouse from whom no valid divorce has occurred

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "bigamy":
   beena marriage, common-law marriage, companionate marriage,
   concubinage, deuterogamy, left-handed marriage, levirate,
   leviration, love match, marriage of convenience, monandry,
   monogamy, monogyny, morganatic marriage, picture marriage,
   polyandry, polygamy, polygyny, trial marriage, trigamy



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

BIGAMY, crim. law, domestic relations. The willful contracting of a second
marriage when the contracting party knows that the first is still
subsisting; or it is the state of a man who has two wives, or of a woman who
has two husbands living at the same time. When the man has more than two
wives, or the woman more than two husbands living at the same time, then the
party is said to have committed polygamy, but the name of bigamy is more
frequently given to this offence in legal proceedings. 1 Russ. on Cr. 187.
     2. In England this crime is punishable by the stat. 1 Jac. 1, c. 11,
which makes the offence felony but it exempts from punishment the party
whose husband or wife shall continue to remain absent for seven years before
the second marriage, without being heard from, and persons who shall have
been legally divorced. The statutory provisions in the U. S. against bigamy
or polygamy, are in general similar to, and copied from the statute of 1
Jac. 1, c. 11, excepting as to the punishment. The several exceptions to
this statute are also nearly the same in the American statutes, but the
punishment of the offence is different in many of the states. 2 Kent, Com.
69; vide Bac. Ab. h. t.; Com. Dig. Justices, Sec. 5; Merlin, Repert. mot
Bigamie; Code, lib. 9, tit. 9, 1. 18; and lib. 5, tit. 5, 1. 2.
     3. According to the canonists, bigamy is three-fold, viz.: (vera,
interpretative, et similitudinaria,) real, interpretative and
similitudinary. The first consisted in marrying two wives successively,
(virgins they may be,) or in once marrying a widow; the second consisted,
not in a repeated marriage, but in marrying (v. g. meretricem vel ab alio
corruptam) a harlot; the third arose from two marriages indeed, but the one
metaphorical or spiritual, the other carnal. This last was confined to
persons initiated in sacred orders, or under the vow Of continence.
Deferriere's Tract, Juris Canon. tit. xxi. See also Bac. Abr. h. t.; 6
Decret, 1. 12. Also Marriage.



The Devil's Dictionary (1881-1906):

BIGAMY, n.  A mistake in taste for which the wisdom of the future will
adjudge a punishment called trigamy.