ship money vs impost

ship money and impost both are nouns.

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

As nouns, impost is a hypernym of ship money; that is, impost is a word with a broader meaning than ship money:

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

Other hypernyms of ship money include custom, customs, customs duty.

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