Answer
Unjust enrichment, under Article 22 of the Civil Code, means that a person who acquires or comes into possession of something at the expense of another, without just or legal ground, must return it. It rests on the maxim that no one may unjustly enrich himself at the expense of another. The action to recover on this basis is called accion in rem verso.
Accion in rem verso is a subsidiary remedy — it is available only when the plaintiff has no other action based on contract, quasi-contract, crime, or quasi-delict, so it does not apply when a valid contract already governs the parties. Its core requisites are that the defendant was enriched, that the plaintiff suffered a corresponding loss, and that there was no just or legal ground for the enrichment.