NOUN (3)

1. the alliance of nations that fought the Axis in World War II and which (with subsequent additions) signed the charter of the United Nations in 1945;

2. in World War I the alliance of Great Britain and France and Russia and all the other nations that became allied with them in opposing the Central Powers;

3. an alliance of nations joining together to fight a common enemy;

The Collaborative International Dictionary of English v.0.48:

Ally \Al*ly"\, n.; pl. Allies. [See Ally, v.]
   1. A relative; a kinsman. [Obs.] --Shak.
      [1913 Webster]

   2. One united to another by treaty or league; -- usually
      applied to sovereigns or states; a confederate.
      [1913 Webster]

            The English soldiers and their French allies.
                                                  --Macaulay.
      [1913 Webster]

   3. Anything associated with another as a helper; an
      auxiliary.
      [1913 Webster]

            Science, instead of being the enemy of religion,
            becomes its ally.                     --Buckle.
      [1913 Webster]

   4. Anything akin to another by structure, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Allies
    n 1: the alliance of nations that fought the Axis in World War
         II and which (with subsequent additions) signed the charter
         of the United Nations in 1945
    2: in World War I the alliance of Great Britain and France and
       Russia and all the other nations that became allied with them
       in opposing the Central Powers
    3: an alliance of nations joining together to fight a common
       enemy