What is a (nautical) knot?

The knot (pronounced not) is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately 1.151 mph. There is no standard abbreviation but kn is commonly used; kt and NMPH are also seen. The knot is a non-SI unit accepted for use with the International System of Units (SI). Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels one minute of geographic latitude in one hour. Etymologically, the term knot derives from counting the number of knots in the line that unspooled from the reel of a chip log in a specific time.

A chip log, also called common log, ship log or just log, is a navigation tool used by mariners to estimate the speed of a vessel through water. The name of the unit knot, for nautical mile per hour, was derived from this method of measurement.

When the navigator wished to determine the speed of his vessel, a sailor dropped the log over the stern of the ship. The log would act as a drogue and remain roughly in place while the vessel moved away. The log-line was allowed to run out for a fixed period of time. The speed of the ship was indicated by the length of log-line passing over the stern during that time.

1 international knot =
    1 nautical mile per hour (by definition)
    1.852 kilometres per hour (exactly)
    0.514 metres per second
    1.151 miles per hour (approximately)