Impost

Noun

 * 1) a tax, tariff or duty etc. that is imposed.  Unlike taxes, tariffs, or duties, the primary purpose of an impost is to dissuade countries from discriminating against the imports or shipping of other nations.  If the trade is non-discriminatory, then no impost is generally levied. It is therefore not intended as a revenue source, but rather as a way to regulate commerce.
 * 2) the weight that must be carried by a horse in a race, the handicap.

Noun

 * 1) the top part of a column or pillar that supports an arch

Thesaurus
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

Etymology 1
through French impôt, from Latin impositus

Etymology 2
through Italian imposta from Latin imposta