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]