A more thorough explanation:

Definition: To become involved in a lawsuit as a third party who was not originally named in the case but has a personal interest in the outcome.

For example, if a company is being sued for polluting a river, a local environmental group may want to intervene in the case to make sure their interests are represented.

There are two types of intervention: intervention as of 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 their 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.

Overall, intervention allows a third party to become involved in a lawsuit to protect their interests and ensure that their voice is heard in the legal process.