A nautical mile (symbol M, NM or nmi) is a unit of distance, set by international agreement as being exactly 1,852 meters (about 6,076 feet). Historically, it was defined as the distance spanned by one minute of arc along a meridian of the Earth (north-south), and developed from the sea mile and the related geographical mile.
It is a non-SI unit (although accepted for use in the International System of Units by the BIPM) which is generally used by navigators at sea and in the air, and also in polar exploration. It remains in use worldwide because of its convenience when working with charts. Most nautical charts use the Mercator projection whose scale varies by about a factor of 6 from the equator to 80° latitude, so charts covering large areas cannot use a single linear scale.The nautical mile is nearly equal to a minute of latitude on a chart, so a distance measured with a chart divider can be roughly converted to nautical miles using the chart’s latitude scale.
The nautical mile is commonly used in international law and treaties, especially regarding the limits of territorial waters.