A plane figure with eight sides is an octagon.
polygon
If they are straight sides, then a polygon. Otherwise, simply "a closed plane figure with 3 or more sides".
A two-dimensional figure, also called a plane or planar figure, is a set of line segments or sides and curve segments or arcs, all lying in a single plane. The sides and arcs are called the edges of the figure. The edges are one-dimensional, but they lie in the plane, which is two-dimensional. The triangle, the pentagon, the hexagon and the circle are just a few plane figures. Prisms and pyramids, for instance, are three-dimension figures.
hendecagon
A plane figure with eight sides is an octagon.
An octagon is a plane figure. It has eight sides, but no real base.
A two-dimensional figure is a polygon. The eight sides means that it uses the prefix octa- (meaning eight) to form the name octagon.
It is a dodecagon
decahedron
A expample of a plane figure with 6 sides and 6 vertices is called a hexagon. hex means 6.
It is a polygon that is a closed plane figure with 3 or more straight lines.
A plane figure with a googol sides is called a googolgon.
The polygon (figure) that contains 8 sides is called an "octagon".
A polygon.
An eight-sided plane figure is an octagon. If the sides are all the same length, and the angles are all equal, it is a regular octagon.
A 12-sided polygon is called a dodecagon. See this website for more names: http://mathforum.org/dr.math/faq/faq.polygon.names.html a plane figure 12 sides