Source : Webster's Revised Unabridged Dictionary (1913)

Biplane \Bi"plane\, n. [Pref. bi- + plane.] (A["e]ronautics)
   An a["e]roplane with two main supporting surfaces one above
   the other.

Biplane \Bi"plane\, a. (A["e]ronautics)
   Having, or consisting of, two superposed planes,
   a["e]rocurves, or the like; of or pertaining to a biplane;
   as, a biplane rudder.

Source : WordNet®

biplane
     n : old fashioned airplane; has two wings one above the other