A polygon is a closed figure made by connecting line segments, where each line segment end connects to only one end of two other line segments.
A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments. These segments are called its edges or sides, and the points where two edges meet are the polygon's vertices (singular: vertex) or corners. An ngon is a polygon with n sides. The interior of the polygon is sometimes called its body. A polygon is a 2dimensional example of the more general polytope in any number of dimensions.
The line segments forming a polygon are called its sides and the endpoints of the line segments are called its vertices. In other words, the meeting point of a pair of sides is called a vertex of the polygon.
In figure (i), ABC is a polygon having three sides AB, BC and CA. Points A, B, C are known as the vertices. As the figure consist of three sides. So, it is called a triangle.
Figure (ii) is a polygon with four sides AB, BC, CD and DA. It has four vertices A, B, C and D. Such a figure is called a quadrilateral. Line segments AC and BD are known as the diagonals.
A closed figure formed by five line segments is known as a pentagon. Figure (iii) is a pentagon with AB, BC, CD, DE and EA as five sides. A, B, C, D and E are five vertices of the pentagon. Line segments AC, AD, BD and BE are diagonals.
Figure (v) is not a polygon as BC is not a line segment.
Polygons are classified according to the number of sides they have.
Name 
No. of sides 
Figures 
Triangle 
3 

Quadrilateral 
4 

Pentagon 
5 

Hexagon 
6 

Heptagon 
7 

Octagon 
8 

Nonagon 
9 

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