Русско-англо-русский юридический словарь

A contract, lease, share, or other right is said to be “onerous” when the obligations attaching to it counter-balance or exceed the advantage to be derived from it, either absolutely or with reference to the particular possessor. Sweet. As used in the civil law and in the systems derived from it, (French, Scotch, Spanish, Mexican) the term also means based upon, supported by, or relating to a good and valuable consideration, i.e. one which imposes a burden or charge in return for the benefit conferred. (Black's Law Dictionary)

An onerous contract (contrato oneroso) is a contract that will include an exchange of consideration (something of value or an exchange of obligations). On the other hand, in gratuitous contracts (contralto gratuity) one party will always carry a burden in favour of the other party (i.e., gift or free loan). (Introduction to Brazilian Law edited by Fabian Deffenti, Weber Oliveira Barrel)

The burden of proof (Latin: onus probandi) is the imperative on a party in a trial to produce the evidence that will shift the conclusion away from the default position to one's own position. (Wikipedia)

Typically, the burden of proof lies with the accuser. Otherwise, anyone could accuse anyone else of anything and then say “prove me wrong.” (Washington Examiner)