Unjust enrichment

Unjust enrichment is a legal concept referring to situations in which one person is enriched at the expense of another in circumstances which the law treats as unjust. Where an individual is unjustly enriched, the law imposes an obligation upon the recipient to make restitution, subject to defences such as change of position. Liability for an unjust (or unjustified) enrichment arises irrespective of wrongdoing on the part of the recipient. The concept of unjust enrichment can be traced to Roman law and the maxim that "no one should be benefited at another's expense": nemo locupletari potest aliena iactura or nemo locupletari debet cum aliena iactura.

The law of unjust enrichment is closely related to, but not co-extensive with, the law of restitution. The law of restitution is the law of gain-based recovery. It is wider than the law of unjust enrichment. Restitution for unjust enrichment is a subset of the law of restitution in the same way that compensation for breach of contract is a subset of the law relating to compensation.

Cases of unjust (or unjustified) enrichment can be examined in the following way:

These questions are a familiar part of the modern English law of unjust enrichment, having been popularised by the writing of Professor Peter Birks and expressly endorsed by English courts. The framework provides a useful taxonomical function in Australian law, though the concept of unjust enrichment has been subject to inconsistent treatment by Australian courts, as discussed below. Stated at this level of abstraction, the framework is a useful grounding for comparative study between common law and civil law jurisdictions.

In systems of law derived from the English common law, the historical core of the law of unjust enrichment lies in quasi-contract. These were common law (as distinct from equitable) claims giving rise to a personal liability to pay the money value of a benefit received from another. Legal scholars from Oxford, Cambridge and Harvard at the turn of the 20th century began to rationalise these disparate actions into a coherent body of law. The principle said to underlie these actions was eventually accepted to be unjust enrichment. Subsequent scholarship has sought to expand the explanatory power of the principle of unjust enrichment and it is now often said (albeit not without controversy) to encompass both common law and equitable claims.

...
Wikipedia