What is the meaning of Impost?

A tax, tariff or duty that is imposed, especially on merchandise.

The weight that must be carried by a horse in a race; the handicap.

The top part of a column, pillar, pier, wall, etc. that supports an arch.

tax

imposed

impost, impost block (projecting block resting on top of a column or embedded in a wall, serving as the base for the springer or lowest voussoir of an arch)

Source: wiktionary.org