Decagon, if a plane figure or decahedron, if a solid.
Dekagon.
A Decagon
An hexagon is polygon that has 6 sides
The name of a 18 sided figure is an octadecagon or octakaidecagon. As you can see the prefix octa- means 8, and deca- means 10. It has 18 sides and 18 vertices,
thirtyagon
quadrilateral
A decagon.
a decagon
Oblong
dodecagon
a pentadecagon