A pentagon is a five-sided polygon, which is a basic shape in geometry. Each of its five sides can be of equal length, making it a regular pentagon, or they can vary in length, creating an irregular pentagon. The sum of the interior angles in a pentagon is 540 degrees. The most famous pentagon is the Pentagon building in Arlington, Virginia, which serves as the headquarters of the United States Department of Defense. Its unique five-sided shape is not only functional but also symbolic of the military's organization and structure.