Pentagon, the BrE AmE
the building in Washington DC from which the army, navy etc of the US are controlled, or the military officers who work in this building. It is called the Pentagon because it has a five-sided shape. On 11 September 2001 it was attacked by ↑ terrorist s who flew a plane filled with passengers into it. 189 people were killed, including the 64 people who were on the plane.