Dictionary.net

• an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator; "all points on the same meridian have the same longitude"

2006 - WordNet 3.0
By Princeton University

• an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator; "all points on the same meridian have the same longitude"

2011 - English Dictionary Database
By DataStellar Co., Ltd