The Paris meridian, also known as the prime meridian or the Greenwich meridian, is an imaginary line that runs from the North Pole to the South Pole, passing through the Royal Observatory in Greenwich, England. It is the starting point for the measurement of longitude on the Earth's surface and is used as a reference point for time zones around the world. The term "meridian" is often used interchangeably with "longitude," "line of longitude," or "meridian line." Other synonyms for the Paris meridian include the Greenwich prime meridian, the zero meridian, or simply the prime meridian.