It's called an Octagon.
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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