履約瑕疵是指債務人履約瑕疵，即履約不符合約定或者約定的條件，致使履約的價值或者效用降低或者喪失的情形。 在中國法律中，效能缺陷被稱為“質量不合格”。
遲延履行是指被執行人在判決、裁定或者其他法律文書規定的期限內未履行支付款項義務的，應當加倍計算遲延履行期間的債務利息。 被執行人未在判決、裁定或者其他法律文書規定的期限內履行其他義務的，應當給予遲延履行的罰款。
拒不履行（毀約）是指債務人在實際能夠履行的情況下故意不履行。 債權是債權，債權的債權是針對債務人的具體行為行使的，而債務人的義務恰恰就是這個具體行為。
履約缺陷、遲交履約和拒履的區別：
效能缺陷是效能不完整的一種形式。 與遲延履行和拒不履行相比，雖然有缺陷的履行不完全，但仍有一些行為可以認為是履行，而遲延履行和拒不履行屬於不履行的消極狀態。
相關雙語規定：
《合同法》第一百一十一條：
質量與約定不符的，按照雙方約定承擔違約責任。 沒有違約責任約定或者約定不明確，依照本法第六十一條的規定不能確定的，受害方可以根據標的物的性質和損失的大小，合理選擇要求對方承擔修理等違約責任。 更換、返工、退貨、降低價格或報酬。
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.
參考雙語例句：
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).
本行或本行的任何實體均不得因超出本行合理控制範圍的原因（包括任何自然災害、天災、公敵、任何軍事、民事或監管機構的行為、任何法律或法規的變更、任何法律或法規的變更）而未能或延遲履行本協議，而被視為違反本協議。 通訊中斷或故障、電源或其他中斷或故障、任何商戶或其他第三方的不履行，或超出銀行（或其所屬的任何實體）合理控制範圍的任何其他原因。
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.
如果由於任何此類原因延遲履行，交貨或裝貨日期將根據需要合理延長，以消除此類延遲的影響。