Definition: Any hypothetical circle that passes through the center of the earth and divides the planet into two hemispheres. The great circle route, following the arc of a great circle on the earth's surface is the shortest route between any two places on earth.
The equator is the only line of latitude that is a great circle; all lines of longitude are great circles. The great circle route can be found by using a string between the two places on a globe. When mapped on a rectangular map projection, a great circle appears to be curved.
Examples:
I presumed we'd fly straight across the Atlantic Ocean to get to London from New York but the pilot used a shorter great circle route that took us just south of Greenland and Iceland.
