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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. In federal cases, Rule 24 of the Federal Rules of Civil Procedure governs intervention.

There are two types of intervention: intervention as a right and permissive intervention. Intervention as of right is when the third party has an unconditional right to enter the litigation based on a statute or when the third party may be bound by the outcome of the case without his interests being adequately represented. In permissive intervention, the court may permit a third party to intervene if the party’s claim shares a common question of law or fact with the existing case and it will not delay the lawsuit or prejudice the original parties’ rights.

[Last updated in June of 2020 by the Wex Definitions Team]