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)

Another civil law classification relates to the advantage each party expects to receive from the agreement. A contract, such as a sale of goods, in which both parties expect to receive an advantage in exchange for the service or item they provide, is considered onerous. A contract where one party provides an advantage without receiving anything in return, such as a donation, is a gratuitous contract. (Civil Law Contracts by Christopher Meldrum)

The contract of deposit may be either onerous or gratuitous. (Louisiana Civil Code)