A stained glass window at the town library is the shape of a regular octagon.

Which of the following describes a regular octagon?

A regular octagon is a bidimensional figure with eight sides and eight vertices. The words "regular" refers to the equivalence between sides, that is, all sides have the same length.

So, a regular octagon can be described as

A figure with eight equal sides. A figure with eight vertices. A polygon. A symmetric figure.

The equivalence between sides makes the regular octagon symmetric. Additionally, this figure is a polygon, because it has 8 sides which enclose a region.