The Collaborative International Dictionary of English v.0.48:

Annexation \An`nex*a"tion\, n. [Cf. F. annexation. See Annex,
   v. t.]
   1. The act of annexing; process of attaching, adding, or
      appending; the act of connecting; union; as, the
      annexation of Texas to the United States, or of chattels
      to the freehold.
      [1913 Webster]

   2.
      (a) (Law) The union of property with a freehold so as to
          become a fixture. Bouvier.
      (b) (Scots Law) The appropriation of lands or rents to the
          crown. --Wharton.
          [1913 Webster]

WordNet (r) 3.0 (2006):

annexation
    n 1: incorporation by joining or uniting [syn: annexation,
         appropriation]
    2: the formal act of acquiring something (especially territory)
       by conquest or occupation; "the French annexation of
       Madagascar as a colony in 1896"; "a protectorate has
       frequently been a first step to annexation"