Русско-англо-русский юридический словарь

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)

lost profits Method of estimating damages in which a seller is compensated for the profit not realized due to a breach of contract by a buyer. (BusinessDictionary.com)

Breach of contract is a legal cause of action in which a binding agreement or bargained-for exchange is not honored by one or more of the parties to the contract by non-performance or interference with the other party's performance. If the party does not fulfill his contractual promise, or has given information to the other party that he will not perform his duty as mentioned in the contract or if by his action and conduct he seems to be unable to perform the contract, he is said to breach the contract. (Wikipedia)

Duran Duran sue own fan club for breach of contract. (The Guardian)