Latin for "friend of the court." A non-party with an interest in the outcome of a pending lawsuit who argues or presents information in support of or against one of the parties to the lawsuit. In many instances, the amicus curiae attempts to draw the court's attention to arguments or information that the parties may not have presented, such as the effects of a particular court ruling on the interests of certain third parties.

An amicus curiae usually presents arguments or information to the court in the form of a brief. Amicus briefs are typically filed at the appellate level, although they also may be filed in lawsuits pending at the trial court level. Generally, an amicus curiae must obtain the court's permission before filing its brief, unless all of the parties consent to the amicus filing.

Amici curiae are not parties to the lawsuit, unless they formally intervene. As a result, an amicus curiae does not need to have standing to bring suit. Further, as a non-party, an amicus curiae normally does not have the rights that parties in a lawsuit have, such as the right to obtain discovery from other parties.