It is neither.
A beit din is a Jewish court. Historically it could address civil, criminal, and religious matters under Jewish law. In modern life, especially outside a sovereign Jewish legal order, it most often handles areas such as conversion, divorce, status, and other disputes or questions voluntarily brought under halakhic authority.
The institution is older than modern rabbinic bureaucracy
Britannica notes that Jewish courts go back to biblical and Second Temple precedents and that the highest ancient court, the Great Bet Din or Sanhedrin, once exercised broad authority. My Jewish Learning makes the practical modern point clearer: in later Jewish life, a beit din usually meant a group of three knowledgeable judges working according to halakhah.
That three-person model is the one most people mean today.
A beit din matters because Jewish law needs institutions, not only books
Texts alone do not decide disputes. Communities need recognized forums to apply them.
That is why the beit din has remained important. Whether the issue is conversion, divorce, financial dispute, or some other question of status and obligation, the beit din functions as a place where Jewish law becomes institutional rather than purely theoretical.
This is also why the term can feel more immediate than outsiders expect. Even in modern Jewish life, some of the most consequential communal questions still run through courts of this kind.