Biplane \Bi"plane\, n. [Pref. bi- + plane.] (A["e]ronautics) An a["e]roplane with two main supporting surfaces one above the other.
Source: Webster's Revised Unabridged Dictionary (1913)
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.