Knot
A Knot is the unit of speed used at sea and in the air: one nautical mile per hour. A ship making 15 knots is covering 15 nautical miles every hour. Speed through the water and speed over the ground are both quoted in knots, as are current and wind speeds.
The name comes from the old practice of measuring speed with a chip log — a line knotted at intervals, streamed astern, with the knots counted as they ran out over a timed period. Modern ships measure speed with an electromagnetic or Doppler speed log.
Speed in knots drives everything commercial about a voyage: the ETA, the fuel burned, the charter-party speed warranty and the demurrage clock. A fraction of a knot in average speed adds up to real money and time over an ocean passage.
Why is speed at sea measured in knots and not km/h?
Because a knot is one nautical mile per hour, and the nautical mile ties directly to latitude on the chart, knots fit naturally with marine navigation and distance measurement.