Definition
The shortest path between two points on the surface of the Earth, formed by the arc of a circle whose center coincides with the center of the Earth. Because the Earth is roughly spherical, this path is curved rather than a straight line on most flat maps, and following it requires a continuously changing true course except along the equator or any meridian.
Plain English
It is the shortest possible flight path between two places on Earth, taking into account that the Earth is round. On a flat map it looks curved, but it is actually the most direct way to get from one point to the other.
Context Anchor
Seen in GPS and long-distance navigation discussions, especially when comparing the path shown by navigation equipment with a straight line drawn on a flat map.
Derivation
A 'great circle' is any circle drawn on a sphere whose center is the same as the center of the sphere itself — making it the largest possible circle on that sphere. Smaller circles drawn on the surface (like lines of latitude other than the equator) are 'small circles.' The term comes from geometry, and 'great' simply means 'largest possible' here, not 'excellent.'
Why Pilots Care
Following the great circle route saves fuel, time, and distance on flights longer than a few hundred miles.
Analogy
Stretch a string tight between two cities on a globe. The path the string traces is the Great Circle route — the shortest distance over the curved surface.
Intuition Check
Great Circle route does not mean a large circular detour. It means the shortest path over the Earth’s curved surface, even if that path looks curved on a flat map.
Example Sentence 1
The GPS calculated a Great Circle route from New York to London, displaying a curved track that arced north over Canada and the North Atlantic.
Example Sentence 2
Because the aircraft followed the great circle route, it arrived with lower fuel burn than a rhumb-line heading would have required.