Definition:Polygon

Definition
A polygon is a closed plane figure made up of an unspecified number of non-crossing straight line segments that join in pairs at their endpoints.

For example:


 * IrregularPolygon.png

Note
The vertices and the sizes of the internal angles of those vertices are frequently referred to by the same letter.

Thus the angle of vertex $A$ is called angle $A$ and denoted $\angle A$.

This is considered by some to be an abuse of notation but its convenience outweighs its disadvantages.