unjust enrichment

1 :  the retaining of a benefit (as money) conferred by another when principles of equity and justice call for restitution to the other party; also :  the retaining of property acquired especially by fraud from another in circumstances that demand the judicial imposition of a constructive trust on behalf of those who in equity ought to receive it — see also quasi contract 1 at contract

2 :  a doctrine that requires an equitable remedy on behalf of one who has been injured by the unjust enrichment of another