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]

WordNet (r) 3.0 (2006):

merger
    n 1: the combination of two or more commercial companies [syn:
         amalgamation, merger, uniting]
    2: an occurrence that involves the production of a union [syn:
       fusion, merger, unification]