In geometry, a monogon, also known as a henagon, is a curve, considered by some as a polygon with one edge and one vertex. It has Schläfli symbol {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 monogonal faces which share one 360ð edge and one vertex. Its dual, a hosohedron, {2,1} has two antipodal vertices at the poles, one 360ð lune face, and one edge () between the two vertices.