Question

what do you mean by prime meridian

Answer 1

A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. Together, a prime meridian and its antimeridian (the 180th meridian in a 360°-system) form a great circle. This great circle divides the sphere, e.g., Earth, into two hemispheres.

Answer 2

The prime meridian is the line of longitude, corresponding to zero degrees and passing through Greenwich, England, from which all the other lines of longitude are calculated.