N-SING
义务；责任;职责
If you say that the onus is on someone to do something, you mean it is their duty or responsibility to do it.
例句
The onus is on the shopkeeper to provide goods which live up to the quality of their description...
店主有义务提供质量与其描述相符的商品。
The onus of proof is reversed in libel actions and placed firmly on the defender.
在诽谤诉讼中，举证责任反过来由辩方全权承担。