Source : Webster's Revised Unabridged Dictionary (1913)

Bilingual \Bi*lin"gual\, a. [L. bilinguis; bis twice + lingua
   tongue, language.]
   Containing, or consisting of, two languages; expressed in two
   languages; as, a bilingual inscription; a bilingual
   dictionary. -- {Bi*lin"gual*ly}, adv.

Source : WordNet®

bilingual
     adj : using or knowing two languages; "bilingual education"

bilingual
     n : a person who speaks two languages fluently