Definition
An intervener (also spelled intervenor) is a person or entity who, not originally a party to a lawsuit or legal proceeding, voluntarily enters that proceeding with the court's permission in order to assert or protect an interest of their own. The intervener becomes a party of record upon the court's grant of leave and may thereafter participate in the litigation to the extent permitted by the court or applicable rules.
Intervention is the procedural mechanism through which an intervener is admitted. Courts distinguish between intervention of right — where the prospective party has a legally protectable interest that the existing litigation may impair — and permissive intervention — where participation is within the court's discretion based on a shared question of law or fact.
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Common Language
Modern common usage (Wiktionary): "One who intervenes."
Historical common usage (Webster's 1913): "A person who assumes a part in a suit between others."
The common meaning tracks the legal meaning more closely than most legal terms, but the gap matters at the edges. In ordinary English, "intervene" suggests any act of stepping in — a bystander preventing a fight, a diplomat interrupting a negotiation. In law, an intervener is not a mere meddler or mediator but a formal party who acquires procedural rights and obligations within active litigation. The court's leave requirement is the critical distinction: an intervener cannot unilaterally insert themselves; admission is a judicial act.
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Common Confusion
INTERVENER vs. AMICUS CURIAE: These are frequently conflated, but the distinction is fundamental. An amicus curiae ("friend of the court") submits argument or information to assist the court but does not become a party and acquires no independent right to litigate the claims. An intervener, once admitted, is a party — subject to discovery, bound by the judgment, and possessed of appellate rights. Researchers reading older decisions should be alert to loose usage; some courts and commentators have described amicus participation in language that implies party status.
INTERVENER vs. THIRD-PARTY DEFENDANT: A third-party defendant is brought into litigation involuntarily by an existing defendant through a claim for contribution or indemnification. An intervener enters voluntarily. The procedural posture — and the strategic implications for the corpus record — differ significantly.
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Recognized Forms
/SUBTYPES
Intervention of Right: The prospective party claims an interest in the subject of the action and shows that disposition of the action may, as a practical matter, impair or impede that interest, and that the existing parties may not adequately represent it. Federal Rule of Civil Procedure 24(a) codifies this standard in U.S. federal practice.
Permissive Intervention: The court may allow a party to intervene when the applicant's claim or defense and the main action share a common question of law or fact. Admission is discretionary and the court may impose conditions. Federal Rule of Civil Procedure 24(b) governs in federal court.
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Why It Matters in Research
The spelling variation — intervener / intervenor — is not merely cosmetic. Legal databases and older digests are inconsistent: American practice and federal rules predominantly use "intervenor," while British-derived sources and some older American texts favor "intervener." A search using only one spelling may miss relevant entries in case digests, treatises, and encyclopedia indexes.
Historical sources treat the leave-of-court requirement as the defining feature but say little about what happens once the intervener is admitted. Modern procedural law adds substantial texture — standing requirements, timeliness of the application, the scope of permissible participation, and whether a denial of intervention is immediately appealable. Researchers using the historical dictionaries as a starting point should supplement with the relevant procedural rules and secondary literature before drawing conclusions about modern practice.
The torts context is a separate, important use of the word. In negligence analysis, an "intervening cause" is a force or act by a third party that comes between the defendant's negligence and the plaintiff's injury. That "intervener" is not a procedural party but a doctrinal actor — a causation concept, not a litigation role. The two uses of the term appear in entirely different bodies of doctrine. A researcher who encounters "intervener" in a torts opinion is almost certainly reading about causation, not procedure.
For administrative law researchers: intervention in agency proceedings follows different rules than civil court intervention, and the concept of "intervener" in public utility or environmental regulatory proceedings carries its own developed body of doctrine distinct from the civil procedure framework.
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Historical Dictionary Support
Both editions of Black's Law Dictionary offer identical, minimal definitions: a person who voluntarily interposes in an action or proceeding with leave of the court. The agreement across editions reflects that the core concept was stable during this period — courts had long recognized the practice of allowing outside parties to appear and be heard.
What the historical dictionaries omit is instructive. Neither edition distinguishes between intervention of right and permissive intervention, a distinction that became the organizing framework of modern procedural codes. Neither addresses standing, timeliness, or the scope of an intervener's participation rights. Webster's 1913 comes closest to the procedural reality by noting that the intervener "assumes a part in a suit," which at least hints at party status, but stops well short of the current doctrinal structure.
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Jurisdictional Note
The substantive framework for intervention is broadly similar across U.S. jurisdictions, but the procedural rules differ. Federal courts apply FRCP 24; state courts apply their own analogs, which vary in how they define adequacy of representation, timeliness requirements, and whether intervention of right is truly non-discretionary. Researchers working in state court records should identify the applicable state rule before mapping federal doctrine onto a state court decision.
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Encyclopedia Cross-Reference
Negligence — Intervening and Superseding Causes, The Law Mind Torts & Personal Injury Encyclopedia
(For the causation doctrine that shares the term "intervener" — distinct from the procedural concept defined here.)
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