In its broadest sense the term "impost" refers to any tax or tribute levied by authority. By usage it has come to have the narrower meaning of a tax or duty imposed on imports. The Supreme Court has recently stated that "imposts and duties" as used in the Constitution "are essentially taxes on the commercial privilege of bringing goods into a country."
Edward L. Barrett, Jr.
"Impost." Encyclopedia of the American Constitution. . Encyclopedia.com. (October 18, 2018). http://www.encyclopedia.com/politics/encyclopedias-almanacs-transcripts-and-maps/impost
"Impost." Encyclopedia of the American Constitution. . Retrieved October 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/politics/encyclopedias-almanacs-transcripts-and-maps/impost