The Prime Meridian is a line of longitude that that spans half of Earth from the North to South Poles. Its longitude is defined as 0°.
The meridian passes through the Royal Observatory and the Headquarters of the Federation Department of Temporal Investigations, both situated in Greenwich, London. In fact, Director Andos' office was right on top of the meridian, her desk and chair postioned so that she could sit with her legs straddling it. (DTI novel: Watching the Clock)