A nautical mile is a unit of distance, or, as physical scientists like to call it, length. It is widely used around the world for maritime and aviation purposes. The derived unit of speed is the knot, defined as one nautical mile per hour.
The nautical mile was historically defined as a minute of arc along a great circle of the Earth. It can therefore be measured on a meridian as change of latitude on a nautical chart. Slight variations in national definitions were settled in 1929, when the International Extraordinary Hydrographic Conference , Monaco adopted a definition of 1 nm = 1,852 metres. The United States adopted it in 1954. The Royal Hydrographic Office of the UK converted in 1970. Prior to the adoption of the international nautical mile, the nautical mile used by the UK was 6080 feet, or 1853.184 metres, and that of the US was 6080.2 ft (1853.249 m).
The BIPM in its SI brochure lists the nautical mile in the table of units "currently accepted" for use with SI without using a symbol, saying in a footnote : "As yet there is no internationally agreed symbol."
The abbreviation "nm" is used. The same abbreviation, "nm", is also used to denote nanometre in the SI though little confusion is generated from this, as the contexts of use are very different. For example, the 'nm' abbreviation is typically seen in listings of aircraft flight range, but listed next to the range in kilometers (km). ([Example ] of nm, km together- in performance section). Also, the large ratio of 1.852 trillion makes confusion unlikely in most cases. The abbreviation nmi is also used.
NMI is also the name of a joint venture responsible for the construction of the Dublin Port Tunnel.
NMI also stands for Non-Maskable Interrupt, a computing term.