Note: Strictly speaking, a polygon does not include its interior (the space inside the polygon).
a polygon  not closed; not a polygon 
not made of line segments; not a polygon 
line segment intersects more than two others; not a polygon 
Polygons are usually classified according to how many sides they have:
A triangle is a polygon with $\,3\,$ sides.  A heptagon is a polygon with $\,7\,$ sides.  
A quadrilateral is a polygon with $\,4\,$ sides.  An octagon is a polygon with $\,8\,$ sides.  
A pentagon is a polygon with $\,5\,$ sides.  A nonagon is a polygon with $\,9\,$ sides.  
A hexagon is a polygon with $\,6\,$ sides.  A decagon is a polygon with $\,10\,$ sides. 
More generally, a polygon with $\,n\,$ sides can be called an $\,n\,$gon.
For example, a polygon with $\,27\,$ sides can be called a $\,27$gon.
When naming polygons, the vertices must be listed in consecutive order. For example, the polygon at right could be named:

More generally, when naming an $\,n$gon, there are $\,n\,$ choices for listing the first vertex.
Then, there are $\,2\,$ choices for the next vertex (moving clockwise or counterclockwise).
The remaining vertices are then completely determined.
Thus, there are $\,2n\,$ choices for the polygon name.
a regular triangle  a regular quadrilateral (a square) 
a regular pentagon  a regular hexagon 
a regular heptagon  a regular octagon  a regular nonagon  a regular decagon 
Note: Every square is a rectangle.
However, not every rectangle is a square.
That is, there exist rectangles that are not squares.
each of these is a rectangle, but NOT a square 
For fun, jump up to WolframAlpha and type in (say)
‘triangle’ or ‘quadrilateral’.
You'll get loads of information!