It is a polygon that is a closed plane figure with 3 or more straight lines.
Circle
No, take a semicircle, it is a plane figure and is open. Or a parabola is also open and a plane figure.
If none of the sides ever cross each other, there cannot be a closed plane figure.
A quadrilateral.
plane figure
A three sided closed plane figure (shape) is a triangle.
It is a polygon that is a closed plane figure with 3 or more straight lines.
a closed plane figure with 3 or more line segments is a polygon
is a closed plane figure
Circle
The distance around a closed plane figure is called the perimeter. The perimeter of a figure can be calculated by adding together the lengths of each side.
No, take a semicircle, it is a plane figure and is open. Or a parabola is also open and a plane figure.
If they are straight sides, then a polygon. Otherwise, simply "a closed plane figure with 3 or more sides".
If none of the sides ever cross each other, there cannot be a closed plane figure.
Any closed figure that is not entirely composed of straight sides, such as an ellipse or a circle.
perimeter