A polygon with 5 sides is a pentagon
yes it can, in fact a polygon can have any amount of sides.. just some don't have names. a polygon is a polygon as long as the lines close at an edge, its a polygon.
It is a plane (2-d) figure defined by N straight sides. An N-sided polygon is used to make statements which depend only on N and not any further details of the shape. For eaxmple, the interior angles of an N-sided polyogon add to (N-2)*180 degrees. This statement is true for all N greater than 2. It does not matter whether you are talking about a triangle, a quadrilateral, a pentagon, etc.