The Collaborative International Dictionary of English v.0.48:

Merger \Mer"ger\, n.
   1. One who, or that which, merges.
      [1913 Webster]

   2. (Law) An absorption of one estate, or one contract, in
      another, or of a minor offense in a greater.
      [1913 Webster]

   3. The combining of two groups into a unified single group
      under a single leadership, with voluntary participation by
      the leaders or management of both groups.
      [PJC]

   4. Specifically: (Business, Finance) The combining of two
      commercial enterprises into a unified single enterprise
      under a single management, with voluntary participation by
      both parties; as, the merger of Daimler-Benz and Chrysler
      into Daimler-Chrysler created a powerful competitor in the
      automobile manufacturing industry. Compare acquisition
      and takeover.
      [PJC]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "merger":
   Anschluss, addition, admixture, affiliation, agglomeration,
   agglutination, aggregation, agreement, alignment, alliance,
   alloyage, amalgamation, articulation, assimilation, association,
   blend, blending, bond, bracketing, cabal, cahoots, cartel,
   centralization, clustering, coadunation, coalescence, coalition,
   colleagueship, collegialism, collegiality, combination, combine,
   combo, comminglement, commingling, commixture, communication,
   composition, comradeship, concatenation, concourse, concurrence,
   confederacy, confederation, confluence, confraternity, congeries,
   conglomeration, conjugation, conjunction, connection,
   consolidation, conspiracy, convergence, copartnership, copartnery,
   copulation, coupling, eclecticism, ecumenism, embodiment,
   encompassment, enosis, federalization, federation, fellowship,
   fraternalism, fraternity, fraternization, freemasonry, fusing,
   fusion, gathering, hookup, identification, immixture, inclusion,
   incorporation, integration, intercommunication, intercourse,
   interfusion, interlarding, interlardment, interlinking,
   interminglement, intermingling, intermixture, joinder, joining,
   jointure, junction, junta, knotting, league, liaison, linkage,
   linking, marriage, meeting, meld, melding, mergence, merging,
   mingling, mixing, mixture, package, package deal, pairing,
   partnership, pluralism, pooling, sodality, solidification,
   sorority, splice, symbiosis, syncretism, syndication, syneresis,
   synthesis, tie, tie-in, tie-up, unification, union, wedding,
   yoking