What is Amicus Curiae ?

Geneva-based Office of the High Commissioner for Human Rights (OHCHR) has announced that it plans to file an application in the Indian Supreme Court, asking to be impleaded in the petitions challenging the Citizenship (Amendment) Act (CAA). 

And the Ministry of External Affairs (MEA) said the United Nations body had no right to file a petition that will ask the court to appoint the Commissioner for Human Rights as an assistant or ‘amicus curiae’ in the case.

In light of these events, what exactly is Amicus Curiae and what is the role ?