The azimuthal coordinate on the surface of a Sphere ( in Spherical Coordinates) or on a Spheroid (in Prolate or Oblate Spheroidal Coordinates). Longitude is defined such that 0° = 360°. Lines of constant longitude are generally called Meridians. The other angular coordinate on the surface of a Sphere is called the Latitude.
The shortest distance between any two points on a Sphere is the so-called Great Circle distance, which can be directly computed from the Latitude and longitudes of two points.
See also Great Circle, Latitude, Meridian, Oblate Spheroidal Coordinates, Prolate Spheroidal Coordinates