Pentagon - building in Washington, D.C. that is shaped like a pentagon (five-sided figure). The Pentagon serves as the headquarters for the Department of Defense and the Joint Chiefs of Staff. Built during World War II, the Pentagon is the largest office building in the world. It holds 20 miles of corridors; 24,000 employees; 4,200 clocks; 685 water fountains; and 87,000 telephones connected by 100,000 miles of cable. Every day, employees and others in the building drink about 30,000 cups of coffee and make about 200,000 telephone calls.