Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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