Why US Department of Defense is called Pentagon?

Photo: Wikipedia 

The Pentagon is the Headquarter of United States Department of Defense. Which is located in Washington DC. This is also called US Department of Defense. The structure of The Pentagon was completed in Almost 2 years (Sep 11,1941 to Jan 15, 1943).
The Architecture Design of THE PENTAGON was designed like a Geometrical Shape of Pentagon which means a shape with 5 Angles.

Thats why US Department of Defense is also called "The Pentagon".