Amicus curiae

Etymology

A Latin phrase meaning "friend of the court," from amicus, "friend," and curiae, the genitive of curia, "court." The literal idea is an outsider offering the court friendly assistance — information or argument it might otherwise lack. Today the amicus brief is a common way for interested third parties to shape important rulings.

noun phrase · /uh-MEE-kus-KYOOR-ee-eye/
A person or group who is not a party to a case but is allowed to advise the court, usually by filing a written brief.
What it names
The reason charities, companies, and scholars can weigh in on a major Supreme Court case even though they are not the ones suing or being sued.
Example
A dozen scientific societies filed an amicus curiae brief warning the justices about the ruling’s effects.
Related idea
An amicus curiae informs the court without a stake in the outcome; a plaintiff or defendant is an actual party whose rights the case decides.