prime meridian

English Thesaurus

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

hypernym	:	line of longitude, meridian,
definition	:	an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator (noun.location)

instance hyponym	:	greenwich meridian,
definition	:	the meridian passing through Greenwich; was internationally adopted as the earth's zero of longitude in 1884 (noun.location)