What is an 8-sided polygon called?

An 8-sided polygon is called an octagon. One common use of the octagon is in the iconic red stop sign that's used throughout the United States.

Octagons are polygons with eight sides. The root word oct- indicates the number eight, as in octopus (which has eight legs). The stop sign form of the octagon has its sides parallel to the x- and y-axes; octagons can also be oriented with its vertices along these axes.

An octagon is a two-dimensional polygon. For three-dimensional shapes (polyhedras), a shape with eight faces is called an octahedron. This include shapes such as a hexagonal prism.