onus vs burden

onus and burden both are nouns.

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

As nouns, onus and burden are synonyms defined as:

Other synonyms of onus include encumbrance, incumbrance, load.

onus (noun)	burden (noun)
an onerous or difficult concern	weight to be borne or conveyed
	the central idea that is expanded in a document or discourse
	an onerous or difficult concern
	the central meaning or theme of a speech or literary work

onus (verb)	burden (verb)
	impose a task upon, assign a responsibility to
	weight down with a load