A prime meridian is a meridian, i.e. a line of longitude, at which the definition of longitude is zero degrees. A prime meridian and its opposite in a 360 degree-system, the 180th meridian, create a great circle.
This great circle separates the sphere into two hemispheres. If one uses directions f East and West from a defined prime meridian, then they can be named Eastern and Western Hemisphere.
A prime meridian is eventually arbitrary, unlike an equator, which is determined by the axis of rotation.
Image Caption: Earth map. Credit: Rodrigocd/Wikipedia