prime meridian, meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the Greenwich meridian. Universal time, the standard basis for determining time throughout the world, is civil time measured at the prime meridian.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
More on prime meridian from Fact Monster:
See more Encyclopedia articles on: Maps and Mapping