不当得利，英文表达为Unjust Enrichment，是指没有法律根据，使他人受到损失而自己获得了利益。不当得利没有法律根据，因此虽属既成事实也不能受到法律的保护，应返还不当利益给受损失的人。这种权利义务关系就是不当得利之债。

其中，取得不当利益的人为得利人（the enriched person），是不当得利之债的债务人，负有返还不当得利的债务；财产受损失的人为受损失的人，是不当得利之债的债权人（creditor），享有请求受益人返还不当利益的债权。

得利人没有法律根据取得不当利益的，受损失的人可以请求得利人返还取得的利益，但是有下列情形之一的除外：

（一）为履行道德义务进行的给付；

（二）债务到期之前的清偿；

（三）明知无给付义务而进行的债务清偿。

相关双语法规：

《民法典》第九百八十六条：得利人不知道且不应当知道取得的利益没有法律根据，取

得的利益已经不存在的，不承担返还该利益的义务。

Where a person enriched does not know or should not have known that the enrichment is without a legal basis, and if the enrichment no longer exists, the person has no obligation to return the benefit thus received.

《民法典》第九百八十七条：得利人知道或者应当知道取得的利益没有法律根据的，受

损失的人可以请求得利人返还其取得的利益并依法赔偿损失。

Where a person enriched knows or should have known that the enrichment is without a legal basis, the aggrieved person may request the enriched person to return the benefit thus received and make compensation for the losses in accordance with law.

《民法典》第九百八十八条：得利人已经将取得的利益无偿转让给第三人的，受损失的人可以请求第三人在相应范围内承担返还义务。

Where a person enriched has gratuitously transfered the benefit received to a third person, the aggrieved person may request the third person to assume the obligation to return the benefit to the corresponding extent.

参考双语例句：

Nicaragua further indicates that it “reserves the right to claim compensation for elements of unjust enrichment consequent upon Colombian possession of the Islands of San Andrés and Providencia as well as the keys and maritime spaces up to the 82 meridian, in the absence of lawful title”.

尼加拉瓜还表示，它“保留就哥伦比亚在没有合法所有权的情况下占有圣安德烈斯岛和普罗维登西亚岛以及直至82度经线以内的礁群和海洋空间而获取的各项不当利益提 出索赔的权利”，并且“保留就尼加拉瓜国籍渔船或尼加拉瓜发照的船只受到的干预提出索赔的权利”。

Until such withdrawal, the licensee has to pay for the use of the licensed intellectual property on the basis of the principle of unjust enrichment an amount equal to the royalties owed under the licence agreement that was rejected.

直至撤销之前，被许可人必须根据不当得利原则，为使用被许可使用的知识产权付费，数额相当于按照被取消的许可协议应当支付的许可使用费。

Under the common counts the pleader need allege only that he is suing for unjust enrichment, for labor performed or for goods sold and delivered or services rendered but not paid for.

根据一般诉讼理由，原告只需宣称他所起诉主张的是（对方的）不当得利，已经付出的劳动，或已经出售并交付的货物，或已经提供服务但对方尚未支付价款。