Theory:

Polygon is a closed two-dimensional shape consisting of only line segments. It does not contain a curved shape. We can also say, polygons are simple closed curves.
A three sided polygon is called a triangle. It is formed by joining three line segments.
 
A four sided polygon is called a quadrilateral. It is formed by joining four line segments.
 
A five sided polygon is called a pentagon. It is formed by joining five line segments.
 
A six sided polygon is called a hexagon. It is formed by joining six line segments.
 
A seven sided polygon is called a heptagon. It is formed by joining seven line segments.
 
A eight sided polygon is called an octagon. It is formed by joining eight line segments.
 
It goes on. We can also form \(n\) sided polygon.
 
Polygon.PNG
Let us see some figures, which are not polygons.
Example:
1Not_pol1.PNG
 
The shape is made up of line segments, but it is not closed. So, this is not a polygon.
 
 
2. Not_pol2.PNG
 
This is a closed shape but not simple. So, this is not a polygon.
 
3. Not_pol3.PNG
 
As per the definition, a polygon does not contain a curved shape. So, this is not a polygon.
 
4. Not_pol4.PNG
 
This is a three dimensional figure. So, this is not a polygon.