Angle
Geometry shape formed by two rays (initial and ending sides of the angle) that share a common endpoint called the vertex. You may name an angle using the vertex, or a point in each ray and the vertex label in the center.

Concave polygon
If a polygon has diagonals that lie outside the polygon then the polygon is concave.

Convex polygon
A convex polygon is any polygon that is not concave.

Decagon
A ten-sided polygon.

Dodecagon
A twelve-sided polygon.

Heptagon
A seven-sided polygon.

Hexagon
A six-sided polygon.

irregular polygon
An irregular polygon is any polygon that is not regular.

N-gon
A polygon with n sides.

Nonagon
A nine-sided polygon.

Octagon
An eight-sided polygon.

Pentadecagon
A 15-sided polygon.

Pentagon
A five-sided polygon.

Polygon
A polygon is a two-dimensional geometric figure with these characteristics: •
It is made of straight line segments.
Each segment touches exactly two other segments, one at each of its endpoints. 
It is closed -- it divides the plane into two distinct regions, one inside and the other outside the polygon.

Regular polygon
A regular polygon has sides that are all the same length and angles that are all the same size.

Quadrilateral
A four-sided polygon.

Septagon
A seven-sided polygon.

Side of a polygon
- a single segment from the union that forms a polygon.

Vertex of a polygon
An endpoint of a segment in a polygon.