Theprime 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.
[technical]
prime meridian in British English
noun
the 0° meridian from which the other meridians or lines of longitude are calculated, usually taken to pass through Greenwich
prime meridian in American English
the meridian from which longitude is measured both east and west; 0° longitude: it passes through Greenwich, England