The North Atlantic Treaty Organization or NATO ( ; ), also called the (North) Atlantic Alliance, is an intergovernmental military alliance based on the North Atlantic Treaty which was signed on 4 April 1949. The organization constitutes a system of collective defence whereby its member states agree to mutual defense in response to an attack by any external party. NATO's headquarters are in Brussels, Belgium, one of the 28 member states across North America and Europe, the newest of which, Albania and Croatia, joined in April 2009. An additional 22 countries participate in NATO's Partnership for Peace, with 15 other countries involved in institutionalized dialogue programs. The combined military spending of all NATO members constitutes over 70% of the world's defence spending.

.nato was a top-level domain in the Domain Name System of the Internet. The domain was added in the late 1980s by InterNIC for the use of NATO, based on the representation that none of the then existing top-level domains (TLDs) adequately reflected their status as an international organization. Soon after this addition, however, the int TLD was created for the use of international organizations, and NATO was persuaded to use nato.int]instead. Without use, the .nato TLD, was deleted in July 1996.