In navigation, a knot is a unit of speed equal to one nautical mile per hour, approximately 1.15 mph; 1.852 km/h or 0.514 m/s.
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.
Ship speeds are usually reported in knots to this day.
See also: Conversion Calculator for Units of SPEED