What is a law based on the customs of a group called?

A law based on the customs of a group is called customary law. Customary law arises from the practices and traditions of a particular community, rather than being enacted through formal legislative processes. It often reflects the values, beliefs, and social norms of the group and can vary significantly between different cultures or societies. Customary law is typically recognized and enforced by the community, and in some cases, it may also be acknowledged by formal legal systems.