Duty and impost have a broad signification which makes them practically synonymous with the general term tax; more generally, however, they are given a narrower meaning according to which they become equivalent to customs or customs dues, that is, to taxes levied upon goods imported from foreign countries.

An excise is an inland tax upon manufacture or retail sale of commodities. It is thus often termed a consumption tax. In the United States the excise taxes are more generally known as internal revenue duties.15

The general power to levy taxes being given, the Constitution enumerates duties, imposts and excises as the classes of taxes which are to be levied uniformly throughout the United States.13