Greenwich meridian 
noun
The prime meridian, which passes through the Royal Observatory at Greenwich in London. It was adopted internationally as the zero of longitude in 1884.Here the zero lines of longitude and latitude - the Greenwich meridian and the equator - bisect....
- Accurate measurements from space have led to another slight shift of the globe's most important line, and the GPS meridian now lies 102.48 metres east of the old Greenwich meridian.
- I stop just outside the front door, and am rather pleased to discover that I live exactly 0°1'2 ‘west of the Greenwich meridian.’