1. an international organization, with headquarters in New York City, formed to promote international peace, security, and cooperation under the terms of the charter signed by 51 founding countries in San Francisco in 1945. Abbr.: UN Cf. General Assembly, Security Council. 2. the nations that signed the joint declaration in Washington, D.C., January 2, 1942, pledging to employ full resources against the Axis powers, not to make a separate peace, etc.