In geometry, a great circle is a circle on a sphere of the maximum possible diameter. A great circle thus has the same diameter as the sphere itself, and cuts the sphere into two equal halves. Examples of great circles on the earth include the equator and all lines of longitude.
The shortest distance between two points on a sphere is a path which follows a great circle. This is why flights from New York to China, instead of going straight east, fly along a great circle, taking them nearly over the North Pole! Because of this length-minimizing property, great circles are called geodesics.