The metric and derived SI unit for speed is metre per second (m/s). Speed is in the term of mathematics a scalar, that is a real number. Velocity (v) on the other hand also has a direction wheras v often is expressed as a vector value or a expressed as a speed and a radian angle to specify direction. These conversions focus on the scalar speed value.
The Imperial and U.S customary system relies on miles and feet when measuring distances which reflects in the units of speed as well.
The knot is defined as the speed when travelling one nautical mile per hour. Knots are used in maritime and air navigation and in meteorology.
The name comes from using a chip log to measure the pace of a vessel at sea. This means throwing a wooden panel attached to a line behind the stern of the wessel and counting the number of knots tied on the line passing by in a certain amount of time elapsed using a 30-second sand-glass. The knots where placed at a distance of 47 feet and 3 inches making it possible to calculate the speed in nautical miles per hour or as it became called, knots.