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Intervention
Intervention is when someone who is not originally a party to a lawsuit asks the court for permission to join the case because they have a direct interest in the outcome.
What it actually is
Intervention is a procedural mechanism allowing a non-party to become a party to an existing lawsuit, typically by filing a motion to intervene. It usually appears early in a case, before trial, and requires the court to decide whether the outsider's interest is sufficiently related to the dispute.
Why it matters in your case
If you are the original party, intervention can bring in an unexpected opponent or ally, potentially changing the strategy, timeline, or scope of the case. If you are the outsider, it is your only way to protect your rights without filing a separate lawsuit.
The common misunderstanding
Many people think intervention means the court will automatically let anyone in who claims an interest, but the court has discretion and often denies intervention if it would delay the case or complicate issues. It is not the same as being a witness or filing an amicus brief.
Get the actual rule
Definitions are orientation; rules are authority
This page explains the concept. When it matters to your case, read the rule that governs it in your court — we hold Texas, federal and Washington court rules word for word, and every state's official resources are on the state pages. If someone cites a case at you, check that it exists first.
Related
Terms that travel with this one
General explanation, not legal advice, and not specific to any state — procedures and deadlines vary by jurisdiction and court. If you can get a lawyer or free legal aid, do: every real option.