Polygons

Polygons
A figure is  a polygon if it is a simple closed figure made up entirely of line segments.
For example, triangle, quadrilateral, pentagon, etc., are all examples of polygon.

If all sides of a polygon are equal and all angles are also equal , then it is called a regular polygon.

Sides, Vertices and Diagonals                                 
The line segments forming a polygon are called its sides.

  The meeting point of a pair of sides is called its vertex.

Sides  and   meet at E, so E is a vertex of the polygon ABCDE. Points B and C are its other vertices. 
 Any two sides with a common end point are called the adjacent sides of the polygon. Sides    

The end points of the same side of a polygon are called the adjacent vertices. Vertices E and D are adjacent, whereas vertices A and D are not adjacent vertices.
The line joining two non-adjacent vertices of a polygon is called a diagonal. Since A and C are non-adjacent vertices, so   is a diagonal.