Princeton's WordNet

1. nautical mile, naut mi, mile, mi, geographical mile, Admiralty mile(noun)

   a former British unit of length equivalent to 6,080 feet (1,853.184 meters); 800 feet longer than a statute mile

2. nautical mile, mile, mi, naut mi, knot, international nautical mile, air mile(noun)

   a unit of length used in navigation; exactly 1,852 meters; historically based on the distance spanned by one minute of arc in latitude