Source : Webster's Revised Unabridged Dictionary (1913)

Ally \Al"ly\, n.
   See {Alley}, a marble or taw.

Ally \Al*ly"\, v. t. [imp. & p. p. {Allied}; p. pr. & vb. n.
   {Allying}.] [OE. alien, OF. alier, F. alier, fr. L. alligare
   to bind to; ad + ligare to bind. Cf. {Alligate}, {Alloy},
   {Allay}, {Ligament}.]
   1. To unite, or form a connection between, as between
      families by marriage, or between princes and states by
      treaty, league, or confederacy; -- often followed by to or
      with.

            O chief! in blood, and now in arms allied. --Pope.

   2. To connect or form a relation between by similitude,
      resemblance, friendship, or love.

            These three did love each other dearly well, And
            with so firm affection were allied.   --Spenser.

            The virtue nearest to our vice allied. --Pope.

   Note: Ally is generally used in the passive form or
         reflexively.

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

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

            The English soldiers and their French allies.
                                                  --Macaulay.

   3. Anything associated with another as a helper; an
      auxiliary.

            Science, instead of being the enemy of religion,
            becomes its ally.                     --Buckle.

   4. Anything akin to another by structure, etc.

Source : WordNet®

ally
     n 1: a friendly nation
     2: an associate who provides assistance; "he's a good ally in
        fight"; "they were friends of the workers" [syn: {friend}]
        [ant: {foe}]
     v : become an ally or associate, as by a treaty or marriage; "He
         allied himself with the Communists"
     [also: {allied}]