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The United Nations (abbreviated UN in English, and ONU in French and Spanish), is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace.

Questions relating to trans-national UN institutions should be tagged with un while questions relating to one or more of the 193 member states should be tagged with the appropriate local tag.

You can learn more about the UN at the Wikipedia.