Impost

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

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