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)

In law, unjust enrichment is where one person is unjustly or by chance enriched at the expense of another, and an obligation to make restitution arises, regardless of liability for wrongdoing. A common example is when a party contracts to provide a service, but the contract is terminated prematurely due to a breach, and the contractor unjustly receives no compensation for partial services rendered.

The concept of unjust enrichment is based upon the Roman legal maxim "no one should be benefited at another's expense" (nemo locupletari potest aliena iactura or nemo locupletari debet cum alien iactura). (Wikipedia)

A Force Majeure clause (French for "superior force") is a contract provision that allows a party to suspend or terminate the performance of its obligations when certain circumstances beyond their control arise, making performance inadvisable, commercially impracticable, illegal, or impossible. (ContractStandards)

The importance of the force majeure clause in a contract, particularly one of any length in time, cannot be overstated as it relieves a party from an obligation under the contract (or suspends that obligation). (Wikipedia)