dihedral

a. [ Gr. di- = di`s- twice + &unr_; a seat, bottom, base, fr. &unr_; to sit. Cf. Diedral. ] 1. Having two plane faces; as, the dihedral summit of a crystal. [ 1913 Webster ]

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

3. (Aëronautics) 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 ]