What is the name for a polygon with five sides?

A polygon with five sides is called a pentagon. The name combines the Greek-derived prefix penta-, meaning “five,” with -gon, a suffix used for figures with angles or sides.

A pentagon has five sides, five vertices and five interior angles. For any simple pentagon, the interior angles add up to 540 degrees. A regular pentagon has five equal sides and five equal angles; each interior angle measures 108 degrees, and its symmetry gives it the familiar star-like geometric relationships seen in many diagrams.

Not every pentagon is regular. A shape can still be a pentagon if its sides or angles differ, provided it is a five-sided polygon. Common mix-ups include the hexagon, which has six sides, and the octagon, which has eight. The Pentagon in Arlington, Virginia, is a famous real-world example of a regular pentagonal building and serves as the headquarters of the United States Department of Defense.