A dihedron is a type of polyhedron, made of two polygon faces which share the same set of n edges.

[1] Dihedra have also been called bihedra,[2] flat polyhedra,[3] or doubly covered polygons.

A dihedron can be considered a degenerate prism whose two (planar) n-sided polygon bases are connected "back-to-back", so that the resulting object has no depth.

Dihedra can arise from Alexandrov's uniqueness theorem, which characterizes the distances on the surface of any convex polyhedron as being locally Euclidean except at a finite number of points with positive angular defect summing to 4π.

This characterization holds also for the distances on the surface of a dihedron, so the statement of Alexandrov's theorem requires that dihedra be considered as convex polyhedra.