BRD is an abbreviation for Business Requirement Document. It is a formal contract between the organization and the client for the development of the specific product. The BRD describes the problems the project is trying to solve and the required outcomes necessary to deliver value.

BRD defines the high-level abstraction of software.
It is a formal document that explains the project requirements given by the client, either written or verbal.
BA creates it after their interaction with clients or stakeholders.