The Collaborative International Dictionary of English v.0.48:

Taxation \Tax*a"tion\, n. [F. taxation, L. taxatio a valuing,
   estimation, from L. taxare. See Tax.]
   1. The act of laying a tax, or of imposing taxes, as on the
      subjects of a state, by government, or on the members of a
      corporation or company, by the proper authority; the
      raising of revenue; also, a system of raising revenue.
      [1913 Webster]

   2. (Law) The act of taxing, or assessing a bill of cost.
      [1913 Webster]

   3. Tax; sum imposed. [R.] --Daniel.
      [1913 Webster]

   4. Charge; accusation. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

taxation
    n 1: charge against a citizen's person or property or activity
         for the support of government [syn: tax, taxation,
         revenue enhancement]
    2: government income due to taxation [syn: tax income,
       taxation, tax revenue, revenue]
    3: the imposition of taxes; the practice of the government in
       levying taxes on the subjects of a state

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "taxation":
   assessment, cess, conscience money, contribution, direct tax, duty,
   graduated taxation, imposition, impost, indirect tax, joint return,
   levy, progressive tax, separate returns, single tax, supertax,
   surtax, tax, tax base, tax dodging, tax evasion, tax exemption,
   tax return, tax structure, tax withholding, tax-exempt status,
   taxable income, tithe, toll, tribute, withholding tax