Polygon

A polygon is any polytope of rank two. These are usually realized in two dimensions, as shapes bounded by straight lines. Polygons can be convex or nonconvex. All polygons are orientable.

Polygons that aren't compounds consist of a single circuit of vertices and edges. They always have the same amount of vertices as edges. As such, polygons may be characterized by any of these numbers. A polygon with $n$ vertices or edges is called an $n$-gon, where $n$ is replaced by the appropriate Greek root. Trigons and tetragons are more commonly called triangles and quadrilaterals.

In Euclidean space
In Euclidean space the simplest possible non-degenerate polygon is a triangle. A digon cannot be realized in Euclidean space with curved or coinciding edges. Geometrically, the regular polygons are those with congruent edges and equal interior angles. There are infinitely many regular convex polygons, one for each number of sides starting from 3. This is unlike the higher-dimensional geometrically regular polytopes, of which there are only finitely many for each dimension.

The possible symmetries of a polygon include no symmetry (scalene triangle), central inversion symmetry (parallelogram), mirror symmetry (isosceles triangle), and dihedral symmetry (square). Polygonal symmetries can exist in higher dimensions, such as pyramidal symmetries, duoprismatic symmetries, and step prism symmetries.

3D Euclidean space
There also exist skew polygons, whose vertices lie in three dimensions or higher. These skew polygons have no defined interior, however they do have straight edges. They appear as the Petrie polygons of polyhedra or as the faces of skew polyhedra. For example, the cube has a regular skew hexagon as its Petrie polygon.

Other spaces
Polygons in hyperbolic space behave very similarly to polygons in Euclidean space. However in spherical, toroidal, and real projective spaces lines can intersect at more than 1 point allowing the non-degenerate digons and monogons.

Abstract polygons
Abstractly there is exactly one polygon for every number greater than 1. Two abstract polygons with the same number of sides are isomorphic. Additionally all abstract polygons are regular and self-dual.

The monogon is not a valid abstract polytope since it violates the diamond condition.

Complexes and maniplexes
For rank 2, complexes and maniplexes are both equivalent to abstract polytopes. All rank 2 complexes are maniplexes, all rank 2 maniplexes are abstract polygons, all abstract polygons are complexes, etc.

However if the definition of a complex is loosened to allow multiple edges, the monogon is a valid complex (and maniplex). This interpretation of the monogon differs substantially from an abstract interpretation, having 2 flags instead of 1.