Meridian

Lines of constant geographic longitude which connect the poles and intersect the equator orthographically; in case of a spherical Earth half a great circle (longitude), in case of an Earth ellipsoid half an ellipse.

The appropriated meridian plane is formed by the current meridian and the pole axis of the Earth.

  
See also
  
 geodesic lines geodesic lines
 parameter lines parameter lines
 Prime Meridian Prime Meridian