The shortest path between two points on a plane is a straight line. On the surface of a sphere, however, there are no straight lines. The shortest path between two points on the surface of a sphere is given by the arc of the great circle passing through the two points. A great circle is defined to be the intersection with a sphere of a plane containing the centre of the sphere.
SESAR Guidance on KPIs and Data Collection Version 1 (D85, Ed. 00.01.01, 2014)
Lexicon Release 2015.1