prime meridian

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A meridian used as a reference line from which longitude east and west is measured.
• noun The meridian passing through Greenwich, England, designated as the zero meridian (0°) by an international conference in 1884.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The reference line at 0° longitude, passing through Greenwich, England, from which longitude east and west is measured.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich longitude in England)