Unjust enrichment is when a person benefits from the loss of another person regardless of whether there was any wrongdoing. Under unjust enrichment, the person who became unjustly enriched is required to pay a fair amount for the enrichment. For example, when a temporary employment agency charges their clients a price that represents exponentially more than the agency pays, the the agency may be found unjustly enriched. If found guilty of unjust enrichment through civil court, they will be required to repay the worker the balance that was unfairly withheld.
The idea of unjust enrichment goes back to Roman times when it was said that a person should not benefit at the expense of another. If a person is not treated fairly within a transaction, they are able to take a case to court to recoup the amount which would make the deal fair.