A knot is a unit of speed equal to one nautical mile per hour. It is 1852 metres per hour precisely (approx 1.15 mph).

The knot is widely used in air and sea navigation.

In many sailing ships, speed was measured by casting the log from the stern. The log was relatively immobile, and attached by line to a reel. Knots placed at a distance of 47 feet 3 inches passed through a sailor's fingers, while another used a 28-second sandglass to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation.