il·le·gal /il-'lē-gəl/ adj: contrary to or in violation of a law: illicit unlawful
an illegal contract
il·le·gal·ly adv

Merriam-Webster’s Dictionary of Law. . 1996.

I adjective actionable, against the law, banned, contrary to law, criminal, exceeding the law, felonious, forbidden, illegitimate, illicit, impermissible, improper, inlicitus, invalid, lawless, not according to law, not allowed, not approved, not authorized by law, not covered by law, not permitted, not valid, outlawed, outside the law, prohibited, prohibited by law, proscribed, punishable, quod contra leges fit, unauthorized, unchartered, unconstitutional, unjustified, unlawful, unsanctioned, unwarrantable, unwarranted, vetitus, without authority, wrongful associated concepts: illegal acts, illegal business, illegal combination, illegal contract, illegal detention, illegal discrimination, illegal force, illegal operation, illegal picketing, illegal possession, illegal practice, illegal purpose, illegal restraint, illegal sale, illegal search and seizure, illegal statute, illegal taking, illegal tax, illegal trade, illegal transaction, illegal use, mala in se, mala prohibita II index felonious, illicit, immoral, impermissible, irregular (improper), lawless, peccant (culpable), unauthorized, unlawful, usurious, wrongful

Burton's Legal Thesaurus. . 2006

Forbidden by law or not according to law; unlawful.

The Essential Law Dictionary. — Sphinx Publishing, An imprint of Sourcebooks, Inc. . 2008.

Against or not authorized by the law. Also called illicit or unlawful.
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Nolo’s Plain-English Law Dictionary. . 2009.

   1) adj. in violation of statute, regulation or ordinance, which may be criminal or merely not in conformity. Thus, an armed robbery is illegal, and so is an access road which is narrower than the county allows, but the violation is not criminal.
   2) status of a person residing in a country of which he/she is not a citizen and who has no official permission to be there.
   See also: alien, illegal immigrant

Law dictionary. . 2013.