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]