The Collaborative International Dictionary of English v.0.48:

Coalition \Co`a*li"tion\, n. [LL. coalitio: cf. F. coalition.
   See Coalesce.]
   1. The act of coalescing; union into a body or mass, as of
      separate bodies or parts; as, a coalition of atoms.
      --Bentley.
      [1913 Webster]

   2. A combination, for temporary purposes, of persons,
      parties, or states, having different interests.
      [1913 Webster]

            A coalition of the puritan and the blackleg. --J.
                                                  Randolph.
      [1913 Webster]

            The coalition between the religious and worldly
            enemies of popery.                    --Macaulay.

   Syn: Alliance; confederation; confederacy; league;
        combination; conjunction; conspiracy; union.
        [1913 Webster]

WordNet (r) 3.0 (2006):

coalition
    n 1: an organization of people (or countries) involved in a pact
         or treaty [syn: alliance, coalition, alignment,
         alinement] [ant: nonalignment, nonalinement]
    2: the state of being combined into one body [syn: coalition,
       fusion]
    3: the union of diverse things into one body or form or group;
       the growing together of parts [syn: coalescence,
       coalescency, coalition, concretion, conglutination]