The Collaborative International Dictionary of English v.0.48:

Illegal \Il*le"gal\, a. [Pref. il- not + legal: cf. F.
   ill['e]gal.]
   Not according to, or authorized by, law; specif., contrary
   to, or in violation of, human law; unlawful; illicit; hence,
   immoral; as, an illegal act; illegal trade; illegal love.
   --Bp. Burnet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

illegal
    adj 1: prohibited by law or by official or accepted rules; "an
           illegal chess move" [ant: legal]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "illegal":
   aberrant, abnormal, abominable, actionable, against the law,
   anarchic, anarchistic, anomic, atrocious, banned, barred,
   black-market, bootleg, chargeable, contraband, contrary to law,
   criminal, delinquent, deviant, disgraceful, evil, felonious,
   flawed, forbade, forbid, forbidden, hardly the thing, hot,
   ignominious, illegitimate, illicit, impermissible, improper,
   inappropriate, incorrect, indecorous, inequitable, infamous,
   iniquitous, interdicted, irregular, justiciable, lawless,
   nonconstitutional, nonlegal, nonlicit, nonpermissible, not done,
   not permitted, not the thing, off limits, off-base, off-color,
   out of bounds, out-of-line, outlaw, outlawed, prohibited,
   proscribed, punishable, ruled out, sacrilegious, scandalous,
   shameful, shameless, sinful, taboo, tabooed, terrible, triable,
   unallowed, unauthorized, unbalanced, unconstitutional,
   under the ban, under-the-counter, under-the-table, undeserved,
   undue, unequal, unequitable, uneven, unfit, unfitting, unjust,
   unlawful, unlicensed, unmeet, unmerited, unofficial, unpermissible,
   unrighteous, unrightful, unsanctioned, unseemly, unstatutory,
   unsuitable, untouchable, unwarrantable, unwarranted, verboten,
   vetoed, wicked, wrong, wrongful



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

ILLEGAL. Contrary to law; unlawful.
     2. It is a general rule, that the law will never give its aid to a
party who has entered into an illegal contract, whether the same be in
direct violation of a statute, against public policy, or opposed to public
morals. Nor to a contract which is fraudulent, which affects the defendant
or a third person.
     3. A contract in violation of a statute is absolutely void, and,
however disguised, it will be set aside, for no form of expression can
remove the substantial defect inherent in the nature of the transaction; the
courts will investigate the real object of the contracting parties, and if
that be repugnant to the law, it will vitiate the transaction.
     4. Contracts against the public policy of the law, are equally void as
if they were in violation of a public statute; a contract not to marry any
one, is therefore illegal and void. See Void.
     5. A contract against the purity of manners is also illegal; as, for
example, a agreement to cohabit unlawfully with another, is therefore void;
but a bond given for past cohabitation, being considered as remuneration for
past injury, is binding. 4 Bouv. Inst. n. 3853.
     6. All contracts which have for their object, or which may in their
consequences, be injurious to third persons, altogether unconnected with
them, are in general illegal and void. Of the first, an example may be found
in the case where a sheriff's officer received a sum of money from a
defendant for admitting to bail, and agreed to pay the bail, part of the
money which was so exacted. 2 Burr. 924. The case of a wager between two
persons, as to the character of a third, is an example of the second class.
Cowp. 729; 4 Camp. 152; 1 Rawle, 42; 1 B. & A. 683. Vide Illicit; Unlawful.