A more thorough explanation:

Definition: Amicus refers to an individual or organization that is not a party to a legal case but is invited or volunteers to advise the court on a matter before it. The term is often used in the phrase "amicus curiae," which means "friend of the court."

Amici curiae, or "friends of the court," can file briefs with courts at the appeals stage in the United States. These briefs may support a party or be neutral in their stance. The role of an amicus is to advise and assist the court on matters of law, bringing to the court's attention certain legal matters that may have been overlooked by the parties involved in the case.

It's important to note that an amicus cannot participate in the litigation as a party or an attorney to a party. There are state and federal laws that govern the participation of an amicus curiae, including rules that specify the content, timing, and manner in which an amicus may participate in a proceeding.

Example: The American Civil Liberties Union (ACLU) filed an amicus brief in support of the plaintiff in the landmark case Brown v. Board of Education. The brief argued that segregation in public schools violated the Equal Protection Clause of the Fourteenth Amendment to the United States Constitution.

This example illustrates how the ACLU, as an amicus curiae, provided legal expertise and arguments to the court in support of the plaintiff's case. The amicus brief helped to shape the court's decision in this historic case.