Monogon

The monogon is a 1-sided polygon. It is highly degenerate. It can be embedding in a spherical geometry, with a single edge that covers an entire great circle and joins a vertex to itself. It is unique compared to other polygons, by having a vertex figure where two different vertices represent the same element.

Most definitions of a polytope do not allow for monogons, as they violate the diamond condition. However, a monogon can be seen as a map on a surface.