English Reference
Noun

onus

n.
C1 Advanced US /ˈoʊnəs/ UK /ˈəʊnəs/ onus UK spelling onuses Law

n.the duty or responsibility to do something. It is the part of a task that you must handle yourself.

n.the duty or burden of proof, responsibility, or action. Often used in legal or formal contexts to describe the obligation placed on a specific party.


SIMPLE

The onus is on the defendant to prove their innocence.

CONTEXTUAL

In many legal cases, the onus of proof lies with the prosecution to establish the defendant's guilt beyond a reasonable doubt.

COMPLEX

The onus of maintaining the historical site fell upon the local council, which struggled to secure the necessary funding for its preservation despite public interest.

He says: "The onus is on Andy Burnham to show he can deliver what he promised.

BBC News· 10 Sept 2026

This is about basic safety standards online for online services and about the onus being on big tech to identify risks on their platform and to mitigate against those risks.

The Guardian· 8 Sept 2026

Etymology tree Proto-Indo-European *h₃enh₂- Proto-Indo-European *-os Proto-Indo-European *h₃énh₂os Proto-Italic *onos Latin onuslbor. English onus Learned borrowing from Latin onus (literally “burden”).

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Level C1 · Advanced
Frequency Occasional
Updated