The Prime Meridian is the meridian (line of longitude) that goes through the Royal Observatory, Greenwich in London.
The answer to your question is,
A prime meridian is a meridian in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its antimeridian form a great circle. This great circle divides the sphere, e.g., Earth, into two hemispheres.
-Mabel <3
