既判力观念渊源于罗马法，大陆法系civil law民事诉讼法都采用了这个概念。是指生效民事判决裁判的诉讼标的对双方当事人和法院所具有的强制性通用力。具体而言，它指的是终局判决一旦获得确定，该判决针对请求所作出的判断就成为规制双方当事人今后法律关系的规范，当双方当事人对同一事项再度发生争执时，就不允许当事人提出与此相矛盾的主张，而且当事人不能对该判断进行争议，法院也不能作出与之相矛盾或抵触的判断。

在英美法系common law，与大陆法系“既判力”观念最相近的是“Res judicata”。据《布莱克法律词典》的解释，是指“已判决的事项或案件。其效力规则是有完全事物管辖权的法院做出的终局判决对当事人及其利害关系人的权利具有决定作用，同时该判决绝对地阻止他们就同一请求和诉因再行起诉”。因此，有人将既判力直译为“Res judicata”。

经搜索，可以看到，维基百科中关于“Res judicata”这一词条也分别对大陆法系中的“Res judicata”及英美法系中的“Res judicata”进行了解释：

部分内容如下：

Common law （英美法系）

In common law jurisdictions, the principle of res judicata may be asserted either by a judge or a defendant.

Once a final judgment has been handed down in a lawsuit, subsequent judges who are confronted with a suit that is identical to or substantially the same as the earlier one will apply the res judicata doctrine to preserve the effect of the first judgment.

Res judicata includes two related concepts: claim preclusion and issue preclusion (also called collateral estoppel or issue estoppel), though sometimes res judicata is used more narrowly to mean only claim preclusion.

Civil law （大陆法系）

The doctrine of res judicata in nations that have a civil law legal system is much narrower in scope than in common law nations.

In order for a second suit to be dismissed on a motion of res judicata in a civilian jurisdiction, the trial must be identical to the first trial in the following manner: (1) identical parties, (2) identical theories of recovery, and (3) identical demands in both trials. In other words, the issue preclusion or collateral estoppel found in the common law doctrine of res judicata is not present in the civilian doctrine. In addition if all else is equal between the two cases, minus the relief sought, there will be no dismissal based on res judicata in a civil law jurisdiction.

A common use of the res judicata principle is to preclude plaintiffs after a class action suit has been settled even on plaintiffs who were not part of the original action because they could have joined that original action.

由上述解释可以发现，英美法系中的“既决力”范围较广，一般包括claim preclusion请求排除 和 issue preclusion挣点排除。而大陆法系中的“既决力”范围较窄，如果想要依据“既决力”驳回对某一罪行的二次诉讼，则第二次审判需要在很多方面与第一次审判保持一致。

以下为相关示例：

很多国家在确立一事不再理原则的同时也规定了再审制度，只是在进行具体的制度设计时，特别注意一事不再理原则的限制作用，在打击犯罪、有错必纠与保障人权、维护既判力之间寻求最佳的平衡点，尽量协调两者间的张力。

Many countries have established the retrial system alongside with the principle of non bis in idem. But they also pay special attention to the restrictive role of the principle when designing specific systems, seeking to strive a good balance between combating crimes, correcting mistakes and protecting human rights and safeguarding res judicata as well as coordinating their tension.