What is a knot, and where did the word come from?

Answer

Sailors threw out a knotted line and counted the knots.

Context

A knot is a speed of one nautical mile per hour. NOAA defines the nautical mile as 1,852 metres, which corresponds to one minute of latitude, and notes that a knot is therefore about 1.15 statute miles per hour. Because it is tied to the Earth's geometry, this unit suits navigation on charts, where latitude markings serve as the distance scale.

In the 17th century, sailors estimated speed with a 'common log': a weighted piece of wood tied to a rope. The log was tossed overboard and the rope paid out freely, with knots tied at regular intervals. A sandglass timed the run, and the number of knots that passed through the hand was the ship's speed. The spacing of knots was chosen so that the count came out directly in nautical miles per hour.

Measurements made this way were crude, since the log could drag and the sandglass timing was approximate, so navigators combined them with compass headings to reckon position. The term persisted after mechanical and electronic logs replaced the rope. Aviation adopted knots too. A frequent error is to say 'knots per hour', which is redundant because the knot already includes the time unit.