You cannot have a 2d cylinder. The 2d cross section will depend on the plane of the cross section.
A twenty sided, 2D figure is called an icosagon.
It is a Heptagon.
Dodecagon http://en.wikipedia.org/wiki/Dodecagon
2d
It's called a "polygon".
2D shapes include squares, triangles, rectangles and circles.
A twenty sided, 2D figure is called an icosagon.
A 2D shape with 9 sides is called a Nonagon.
A ten-sided 2D polygon is known as a decagon.
The 2D name for an octagonal prism is an octagon. An octagonal prism consists of two parallel octagonal bases connected by rectangular faces, but when referring to its 2D representation, it is simply an octagon.
A rectangle is one of many names in the classification of 2D shapes as are polygons
Octagon
Square
Corners.
hexagon
Hexagon
Tetradecagon.