Amicus curiae

An amicus curiae is someone, not a party to a case, who volunteers to offer information to assist a court in deciding a matter before it.
Amicus curiae islegal Latin and literally means "friend of the court ''
The information provided may be -
 a legal opinion in the form of a brief(which is called an amicus brief when offered by an amicus curiae)or
 a testimony that has not been solicited by any of the parties or
 a learned treatise on a matter that bears on the case. 

The decision on whether to admit the information lies at the discretion of the court.