Unjust enrichment: gain without legal or contractual basis at the expense of others. Among them, the beneficiary is the debtor and has the obligation to return the unjust enrichment. The injured person is the creditor and has the right to request the beneficiary to return the undeserved gain.
Xiao Mei, a cashier at the mall, giving the customer a change of 100 yuan instead of 10 yuan is an example of unjust enrichment according to the above definition.