Intervention在这里指参加诉讼；加入诉讼，有时也可直接译为介入。其英文释义如下：

The entry into a lawsuit by a third party into an existing civil case who was not named as an original party but has a personal stake in the outcome. The nonparty who intervenes in a case is called an intervenor. The intervener joins the suit by filing a motion to intervene. An intervenor can join the side of the plaintiff, defendant, or as adverse to both the plaintiff and defendant. 

相关的intervener指参加诉讼人；加入诉讼人。自愿并经法庭许可而加入到某一诉讼中来，并成为该诉讼的一方当事人，以维护自己的权利或利益的人。

“intervention of right”和“permissive intervention”属介入诉讼的两种类型，一种是有权介入，另一种是许可介入。其源于美国《联邦民事诉讼规则》（Federal Rules of Civil Procedure） 第24条（Rule 24. Intervention），相关定义如下：

有权介入Intervention of Right. 

On timely motion, the court must permit anyone to intervene who:

(1) is given an unconditional right to intervene by a federal statute; or

(2) claims an interest relating to the property or transaction that is the subject of the action, and is so situated that disposing of the action may as a practical matter impair or impede the movant's ability to protect its interest, unless existing parties adequately represent that interest.

许可介入Permissive Intervention.

(1) In General. On timely motion, the court may permit anyone to intervene who:

(A) is given a conditional right to intervene by a federal statute; or

(B) has a claim or defense that shares with the main action a common question of law or fact.

(2) By a Government Officer or Agency. On timely motion, the court may permit a federal or state governmental officer or agency to intervene if a party's claim or defense is based on:

(A) a statute or executive order administered by the officer or agency; or

(B) any regulation, order, requirement, or agreement issued or made under the statute or executive order.

根据上述定义可知，有权介入规定，如果案外人缺席时可能会实际削弱或妨碍其保护利益的能力，那么该案外人介入诉讼或者被并入诉讼就是适当的。而许可介入则指法院在某些情况下允许某人或某政府官员或机构加入诉讼。

无论是有权介入还是许可介入，都需要介入方及时提起介入申请（motion to intervene）。