In geometry, a monogon, also known as a henagon, is a polygon with one edge and one vertex.

[1] In Euclidean geometry a monogon is a degenerate polygon because its endpoints must coincide, unlike any Euclidean line segment.

Most definitions of a polygon in Euclidean geometry do not admit the monogon.

In spherical geometry, a monogon can be constructed as a vertex on a great circle (equator).

This forms a dihedron, {1,2}, with two hemispherical monogonal faces which share one 360° edge and one vertex.