A twenty sided, 2D figure is called an icosagon.
An eight sided figure is called an octagon.
An 11 sided figure is called a hendecagon, an undecagon, or a 11-gon.
A 5-sided figure is called a pentagon.
A six sided figure is called a hexagon, as "hex" means "six."
A 20-sided flat figure (polygon) is called an icosagon. If it is a solid figure--three-dimensional--it is an icosahedron.
Icosagon is a 20 sided figure.
A twenty-eight sided figure is called an Icosikaioctagon. The word icos refers to the number twenty and the word octagon refers to eight. When combined, they form the word Icosikaioctagon.
A twenty sided, 2D figure is called an icosagon.
Dodecahedron
An eight sided figure is called an octagon.
A 3-sided figure is called a triangle.
A four sided figure is called a quadrilateral.
A 12-sided figure is called a dodecagon.
A seven sided figure is called a heptagon.
A six-sided plane figure is called a hexagon.
A six-sided plane figure is called a hexagon.