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"

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "annexation":
   abstraction, accession, accessory, accompaniment, addenda,
   addendum, additament, addition, additive, additory, additum,
   adhesive, adjunct, adjunction, adjuvant, affixation, agglutination,
   angary, annex, annexure, appanage, appendage, appendant,
   appropriation, appurtenance, appurtenant, attachment, augment,
   augmentation, binding, bond, boosting, clasping, coda,
   collectivization, commandeering, communalization, communization,
   complement, concomitant, confiscation, continuation, conversion,
   conveyance, corollary, distraint, distress, embezzlement,
   eminent domain, execution, expropriation, extension, extrapolation,
   fastener, fastening, filching, fixture, fraud, garnishment,
   girding, graft, hooking, impoundment, impressment, increase,
   increment, joining, junction, juxtaposition, knot, lashing, levy,
   liberation, lifting, ligation, nationalization, offshoot, pendant,
   pilferage, pilfering, pinching, poaching, prefixation,
   reinforcement, right of angary, scrounging, sequestration,
   shoplifting, side effect, side issue, snatching, sneak thievery,
   snitching, socialization, splice, stealage, stealing, sticking,
   suffixation, superaddition, superfetation, superjunction,
   superposition, supplement, supplementation, swindle, swiping,
   tailpiece, theft, thievery, thieving, tieing, undergirding,
   uniting, zipping



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

ANNEXATION, property. The union of one thing to another.
     2. In the law relating to fixtures, (q.v.) annexation is actual or
constructive. By actual annexation is understood every movement by which a
chattel can be joined or united to the freehold. By constructive annexation
is understood the union of such things as have been holden parcel of the
realty, but which are not actually annexed, fixed, or fastened to the
freehold; for example, deeds, or chattels, which relate to the title of the
inheritance. Shep. Touch. 469. Vide Anios & Fer. on Fixtures, 2.
     3. This term has been applied to the union of one country, to another;
as Texas was annexed to the United States by the joint resolution of Congress

of larch 1, 1845., See Texas.