from
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]