IMPOST, a tax or tribute, and particularly a duty levied on imported or exported merchandise. In architecture, a moulding, series of mouldings, projecting band, carved ornament or project ing block, marking the spring of an arch, and placed at the top of the pier or section of wall that carries the arch. In Byzantine work the block sometimes placed between the capital of a column and the arches which it carries is known as an impost block.