A line of constant Longitude on a Spheroid (or Sphere). More generally, a meridian of a Surface of Revolution is the intersection of the surface with a Plane containing the axis of revolution.
See also Latitude, Longitude, Parallel (Surface of Revolution), Surface of Revolution
Gray, A. Modern Differential Geometry of Curves and Surfaces. Boca Raton, FL: CRC Press, p. 358, 1993.