Historically, the nautical mile was defined as ...
one minute of latitude
one hour cruise on average wind
1/12 distance from Calais to London
1000 English yards
In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as precisely 1,852 meters. The United States nautical mile was defined as 6,080.20 feet (1,853.24 m) based in the Mendenhall Order foot of 1893. It was abandoned in favour of the international nautical mile in 1954. The Imperial nautical mile, often called an Admiralty mile, was defined by its relation to the Admiralty knot, and was abandoned in 1970.