onus vs imposition

onus and imposition both are nouns.

Word	Noun	Adjective	Verb	Adverb
onus	Yes	No	No	No
imposition	Yes	No	No	No

As nouns, imposition is a hyponym of onus; that is, imposition is a word with a more specific, narrower meaning than onus:

Other hyponyms of onus include dead weight, fardel, pill.

onus (noun)	imposition (noun)
an onerous or difficult concern	the act of imposing something (as a tax or an embargo)
	an uncalled-for burden