The Collaborative International Dictionary of English v.0.48:

Impost \Im"post\, n. [OF. impost, F. impot, LL. impostus, fr. L.
   impostus, p. p. of imponere to impose. See Impone.]
   1. That which is imposed or levied; a tax, tribute, or duty;
      especially, a duty or tax laid by goverment on goods
      imported into a country.
      [1913 Webster]

            Even the ship money . . . Johnson could not
            pronounce to have been an unconstitutional impost.
                                                  --Macaulay.
      [1913 Webster]

   2. (Arch.) The top member of a pillar, pier, wall, etc., upon
      which the weight of an arch rests.
      [1913 Webster]

   Note: The impost is called continuous, if the moldings of the
         arch or architrave run down the jamb or pier without a
         break.

   Syn: Tribute; excise; custom; duty; tax.
        [1913 Webster]

WordNet (r) 3.0 (2006):

impost
    n 1: money collected under a tariff [syn: customs, customs
         duty, custom, impost]
    2: the lowest stone in an arch -- from which it springs [syn:
       springer, impost]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "impost":
   assessment, blackmail, call, call for, cess, claim,
   conscience money, contribution, demand, demand for, direct tax,
   draft, drain, duty, exaction, extortion, extortionate demand,
   graduated taxation, heavy demand, imposition, indent, indirect tax,
   insistent demand, joint return, levy, nonnegotiable demand, notice,
   order, progressive tax, requirement, requisition, rush, rush order,
   separate returns, single tax, supertax, surtax, tariff, tax,
   tax base, tax dodging, tax evasion, tax exemption, tax return,
   tax structure, tax withholding, tax-exempt status, taxable income,
   taxation, taxing, tithe, toll, tribute, ultimatum, warning,
   withholding tax