The Collaborative International Dictionary of English v.0.48:

Ally \Al"ly\, n.
   See Alley, a marble or taw.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            O chief! in blood, and now in arms allied. --Pope.
      [1913 Webster]

   2. To connect or form a relation between by similitude,
      resemblance, friendship, or love.
      [1913 Webster]

            These three did love each other dearly well,
            And with so firm affection were allied. --Spenser.
      [1913 Webster]

            The virtue nearest to our vice allied. --Pope.
      [1913 Webster]

   Note: Ally is generally used in the passive form or
         reflexively.
         [1913 Webster]

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):

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

Moby Thesaurus II by Grady Ward, 1.0:

210 Moby Thesaurus words for "ally":
   accessory, accomplice, act in concert, act together, adjunct,
   affiliate, alter ego, amalgamate, analogon, analogue, apply,
   archduchy, archdukedom, associate, band, band together,
   be in cahoots, be in league, bedfellow, bind, body politic,
   bracket, brother, brother-in-arms, buffer state, bunch, bunch up,
   cabal, captive nation, cement a union, centralize, chieftaincy,
   chieftainry, city-state, close copy, close match, club,
   club together, coact, coadjutor, coalesce, cognate, cohort,
   collaborate, collaborator, colleague, collude, colony, combine,
   come together, commonweal, commonwealth, companion, compatriot,
   compeer, complement, comrade, concert, concord, concur,
   confederate, confrere, congenator, congener, connect, consociate,
   consolidate, consort, conspire, cooperate, coordinate, correlate,
   correlative, correspondent, counterpart, country, county, couple,
   crony, do business with, domain, dominion, draw a parallel, duchy,
   dukedom, earldom, empery, empire, equate, equivalent, federalize,
   federate, fellow, fellow member, free city, friend, fuse, gang,
   gang up, get heads together, get together, go in partners,
   go in partnership, go partners, grand duchy, hang together,
   harmonize, hold together, hook up, hook up with, identify, image,
   interrelate, join, join forces, join fortunes with, join in,
   join together, join up, join up with, join with, keep together,
   kindred spirit, kingdom, land, league, league together, like,
   likeness, link, make common cause, mandant, mandate,
   mandated territory, mandatee, mandatory, marry, mate, merge,
   nation, nationality, near duplicate, obverse, organize, pair,
   pair off, parallel, parallelize, partner, pendant, picture,
   play ball, polis, polity, possession, power, principality,
   principate, protectorate, province, pull together,
   puppet government, puppet regime, put heads together, realm,
   reciprocal, reciprocate, relate, relativize, republic, satellite,
   second self, seneschalty, settlement, side, similitude, simulacrum,
   sister, soul mate, sovereign nation, stand together, stand up with,
   state, such, suchlike, sultanate, superpower, tally, team up,
   team up with, team with, territory, the like of, the likes of,
   throw in together, throw in with, tie, tie in, tie in with, tie up,
   tie up with, toparchia, toparchy, twin, unionize, unite,
   unite efforts, unite with, wed, work together



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ALLY, international law. A power which has entered into an alliance with
another power. A citizen or subject of one of the powers in alliance, is
sometimes called an ally; for example, the rule which renders it unlawful
for a citizen of the United States to trade or carry on commerce with an
enemy, also precludes an ally from similar intercourse. 4 Rob. Rep. 251; 6
Rob. Rep. 406; Dane's Ab, Index, h.t.; 2 Dall. 15.