瑕疵履行（defective performance），指债务人虽然履行，但其履行存在瑕疵，即履行不符合规定或约定的条件，致减少或丧失履行的价值或效用的情形。瑕疵履行在中国立法中称为“质量不符合约定”。

迟延履行（delay in performance），是指被执行人未按判决、裁定和其他法律文书指定的期间履行给付金钱义务的，应当加倍支付迟延履行期间的债务利息。被执行人未按判决、裁定和其他法律文书指定的期间履行其他义务的，应当支付迟延履行金。

拒绝履行（repudiate），是指债务人能够实际履行而故意不履行。债权为请求权，债权人的请求权是针对于债务人的特定的行为行使的，债务人的义务也正是此特定的行为，此债权人得为请求及债务人所应实行的行为即为给付。

瑕疵履行、延迟履行和拒绝履行的区别：

瑕疵履行属于不完全履行的一种形态。与迟延履行、拒绝履行相比，瑕疵履行虽然履行不完全，但尚有可认为履行的行为，而迟延履行、拒绝履行则属于无履行的消极状态。

相关双语法规：

《合同法》第一百一十一条：

质量不符合约定的，应当按照当事人的约定承担违约责任。对违约责任没有约定或者约定不明确，依照本法第六十一条的规定仍不能确定的，受损害方根据标的的性质以及损失的大小，可以合理选择要求对方承担修理、更换、重作、退货、减少价款或者报酬等违约责任。

Where the quality fails to satisfy the agreement, the breach of contract damages shall be borne in the manner as agreed upon by the parties. Where there is no agreement in the contract on the liability for breach of contract or such agreement is unclear, nor can it be determined in accordance with the provisions of Article 61 of this Law, the damaged party may, in light of the nature of the subject matter and the degree of loss, reasonably choose to request the other party to bear the liabilities for the breach of contract such as repairing, substituting, reworking,returning the goods, or reducing the price or remuneration.

参考双语例句：

The Practice will not, nor will any entity that is part of the Practice, be deemed to be in breach of this Agreement for any failure or delay in performance caused by reasons beyond its reasonable control, including any natural calamity, act of God or a public enemy, act of any military, civil or regulatory authority, change in any law or regulation, disruption or outage of communications, power or other, failure to perform by any supplier or other third party, or any other cause beyond the reasonable control of the Practice (or any entity that is part of the Practice).

本行和属于本行一部分的任何实体不应因其合理控制外的原因（包括任何自然灾害、天灾、公敌、任何军事、民事 或监管机关的行为、任何法律或法规的变更、通讯、电力或其他中断或故障、任何供应商或其他第三方不履行、或超出本行（或作为其部分的任何实体）合理控制的任何其他原因）导致的不履行或迟延履行被视为违反本协议。

In the event of delay in performance due to any such cause, the date of delivery or shipment will be extended by a period of time reasonably necessary to overcome the effect of such delay.

如果由于任何此类原因导致履行延迟，交付或装货的日期将会按需合理延长一段时间以消除此类延迟带来的影响。