The Collaborative International Dictionary of English v.0.48:

Dihedral \Di*he"dral\, a. [Gr. di- = di`s- twice + ? a seat,
   bottom, base, fr. ? to sit. Cf. Diedral.]
   1. Having two plane faces; as, the dihedral summit of a
      crystal.
      [1913 Webster]

   2. Of a kite or an a["e]roplane, having wings that make with
      one another a dihedral angle, esp. when the angle between
      the upper sides is less than 180[deg].
      [Webster 1913 Suppl.]

   3. (A["e]ronautics) Of wing pairs, inclined at an upward
      angle to each other.
      [Webster 1913 Suppl.]

   Dihedral angle, the angular space contained between planes
      which intersect. It is measured by the angle made by any
      two lines at right angles to the two planes.
      [1913 Webster]