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.
According to the above definition, Xiao Zhang winning 200 yuan during the Spring Festival by playing mahjong with relatives at home is unjust enrichment.