Real Dictionary

NATO
proper noun: NATO is an international organization which consists of the USA, Canada, Britain, and other European countries, all of whom have agreed to support one another if they are attacked. NATO is an abbreviation for 'North Atlantic Treaty Organization'.

NATO in American English
North Atlantic Treaty Organization
noun: an organization formed in Washington, D. C. (1949), comprising the 12 nations of the Atlantic Pact together with Greece, Turkey, and the Federal Republic of Germany, for the purpose of collective defense against aggression

NATO in British English
noun acronym for: North Atlantic Treaty Organization, an international organization composed of the US, Canada, Britain, and 27 European countries: established by the North Atlantic Treaty (1949) for purposes of collective security