What is the name for a figure with eight sides?

A figure with eight sides is called an octagon. The name comes from Greek roots: octa means “eight,” while gon refers to an angle or corner. In geometry, an octagon therefore has eight sides, eight vertices, and eight interior angles.

A regular octagon has eight equal sides and eight equal angles. Each interior angle in a regular octagon measures 135 degrees, and the sum of all its interior angles is 1,080 degrees. Stop signs are a familiar real-world example of regular octagons.

The most common mix-ups are with nearby polygon names. A hexagon has six sides, a nonagon has nine, and a decagon has ten. An octagon does not have to be regular: any closed polygon with eight straight sides qualifies, even if its sides or angles are unequal.