impost vs ship money

impost and ship money both are nouns.

Word	Noun	Adjective	Verb	Adverb
impost	Yes	No	No	No
ship money	Yes	No	No	No

As nouns, ship money is a hyponym of impost; that is, ship money is a word with a more specific, narrower meaning than impost:

impost: money collected under a tariff
ship money: an impost levied in England to provide money for ships for national defense

impost (noun)	ship money (noun)
the lowest stone in an arch -- from which it springs	an impost levied in England to provide money for ships for national defense
money collected under a tariff