proportional tax vs taxation

proportional tax and taxation both are nouns.

Word	Noun	Adjective	Verb	Adverb
proportional tax	Yes	No	No	No
taxation	Yes	No	No	No

As nouns, taxation is a hypernym of proportional tax; that is, taxation is a word with a broader meaning than proportional tax:

proportional tax: any tax in which the rate is constant as the amount subject to taxation increases
taxation: charge against a citizen's person or property or activity for the support of government

Other hypernyms of proportional tax include revenue enhancement, tax.

proportional tax (noun)	taxation (noun)
any tax in which the rate is constant as the amount subject to taxation increases	the imposition of taxes; the practice of the government in levying taxes on the subjects of a state
	charge against a citizen's person or property or activity for the support of government
	government income due to taxation