burden vs onus

burden and onus both are nouns.

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

As nouns, burden and onus are synonyms defined as:

Other synonyms of burden include encumbrance, incumbrance, load.

burden (noun)	onus (noun)
weight to be borne or conveyed	an onerous or difficult concern
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

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