Unjust enrichment

The retention of a benefit conferred by another, that is not intended as a gift and is not legally justifiable, without offering compensation, in circumstances where compensation is reasonably expected. 

The elements of a cause of action for unjust enrichment are:  the enrichment of the party accused of unjust enrichment; that such enrichment was at the expense of the party seeking restitution; and the circumstances were such that in equity and good conscience restitution should be made.  An additional requirement is that the party accused of unjust enrichment must know of the benefit conferred; to ensure that the benefit was not foisted on the recipient and is something for which compensation is reasonably expected.

Recovery on a theory of unjust enrichment typically occurs where there was no contract between the parties, or a contract turns out to be invalid.  See quasi-contract.