Angle
Geometric 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.

Area
The amount of space taken up in a plane by a figure.

Circle
The set of points on a plane at a certain distance (radius) from a certain point (center); a polygon with infinite sides

Circumscribed circle or circumcircle of a polygon: A polygon is circumscribed about a circle if all the sides are tangent to the circle, and a circle is circumscribed about a polygon if all the vertices of this are on the circumference of the circle.

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.

Inscribed planar shape or solid: A polygon is inscribed in a circle if the vertices of a polygon inside a circle are on the circumference of the circle; a circle is inscribed to a polygon if all the sides of the polygon are tangent to the circle in the interior of the 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.

Radii
Plural form of radius.

Radius
The segment whose endpoints are any point on a circle or sphere and its center; the length of that segment

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.