A great circle is the line of intersection of the surface of a sphere and any plane that passes through the center of the sphere. The shortest route between two points on a sphere, such as Earth, is along the great circle that connects these points. A great circle is a geodesic.

Great circle routes are of great importance in air travel. Great circle navigation is aided by use of maps drawn to a gnomonic projection, where the center of perspective is the center of Earth. On such maps, great circles appear as straight lines.