根本违约（Fundamental breach of contract ），是指当事人一方迟延履行债务或者其他违约行为将导致合同相对方合同目的落空的违约方的严重违约行为。根据《合同法》第94条之规定可以看出，当事人一方迟延履行其主要债务，还不能称之为根本违约，只有当其迟延履行行为致使相对方合同目的不能实现时，才可将之称为根本违约。 根本违约的构成要件是一般违约的构成要件，加上因违约行为导致的合同目的不能实现，其法律效果是当一方根本违约时，另一方当事人可以解除合同并要求对方承担违约责任。

根本违约是从英国普通法上发展出来的一种制度，其影响力之大在联合国国际货物销售合同公约、国际商事合同通则、欧洲合同法原则中均有体现。其构成要件总体上存有条款主义与结果主义，我国立法上应采取结果主义的判断标准，同时在具体的判断上可参照所违反义务的类型标准。在迟延履行、履行不能、不完全履行、先期违约类型场合，根本违约都有特定构成标准。根本违约一旦构成，产生的法律效果有二：一是债权人可以解除债权；二是对合同解除权的限制。我国新颁布的合同法采纳了根本违约制度，一方面作为一种法定解除权发生的事由，另一方面实际上又对解除权的行使予以必要的限制。

为了更好理解该词，以下是其英文释义：Fundamental breach of contract is a controversial concept within the common law of contract. The doctrine was, in particular, nurtured by Lord Denning MR, but it did not find favour with the House of Lords.

Whereas breach of condition is a serious breach that “denies the plaintiff the main benefit of the contract”, fundamental breach was supposed to be even worse, with the result that any exemption clause limiting the defendant's liability would automatically become void and ineffective. Also, whereas breach of condition gives the plaintiff the option to repudiate, fundamental breach automatically discharges the entire contract.

双语例句：

（1）为顺利开展双方合作，乙方同意向甲方支付保证金人民币【五万】元整。乙方应当于本合同签署并生效之日起【15】个工作日内足额支付，该保证金足额支付完毕后，甲乙双方合作正式开展。若超出前述期限乙方未足额支付保证金的，本合同自乙方逾期缴纳之日起自动终止，自始不发生法律效力。

In order to successfully carry out the cooperation between Party A and Party B, Party B agrees to pay Party A a security deposit of RMB [50,000]. Party B shall pay in full within [15] working days from the date when this Contract is signed and takes effect. After the security deposit is paid in full, the cooperation between the two Parties will be officially started. If Party B fails to pay the security deposit in full within the aforementioned time limit, this Contract shall be automatically terminated from the date of late payment by Party B, and will have no legal effect from the outset.

（2）本合同终止之日起（包括但不限于一方违约终止、双方协商解除、合同到期未续约等情形）【1】个自然月后，若乙方不存在任何违反本合同约定的情况的，该笔保证金将全额无息返还。

If, within [1] natural month from the date on which this Contract is terminated (including but not limited to the termination due to one Party’s breach of contract, cancellation by both Parties through negotiation, and non-renewal of the Contract upon expiration, etc.), there is no breach of this contract by Party B, the security deposit will be refunded to Party B in full without interest.