Merriam-Webster Collegiate Dictionary - nautical mile

 noun  Date: 1834 any of various units of distance used for sea and air navigation based on the length of a minute of arc of a great circle of the earth and differing because the earth is not a perfect sphere: as  a. a British unit equal to 6080 feet (1853.2 meters)  b. an international unit equal to exactly 1852 meters (6076.115 feet or 1.15 statute miles) used officially in the United States since July 1, 1954