"As the crow flies" is a phrase used to describe the most direct route between two points on a planetary body. It is most often used to differentiate distance along this route from that along a less direct route, such as a road or railway, and is generally the same as the great circle route. Some people use the variation "by the crow flies."
An example would be the distance between Key West and Pensacola, at the two opposite ends of Florida, in the United States. The most direct route by road is about 792 miles (1,275 kilometers); but a measurement directly across the Gulf of Mexico is only 524 miles (843 kilometers)—as the crow flies.
The phrase is attested in print as early as 1800.