Pentagon

The Pentagon was a building housing the United States Department of Defense. Bill and Nardole visited it in the Monks' virtual reality. (TV: Extremis)

It was one of the organisations that Tanya Adeola attempted to hack as a hobby. (PROSE: The Stone House)

Behind the scenes
Pentagon is the building housing the US Department of the Defense headquarters. The name of the building is often colloquially used for the organisation itself.