Learn in the textbookBusiness Requirements Definition — Functional / Non-functional Requirements and Agreement
DefinitionBusiness requirements definition (業務要件定義) is the work of taking the problems found by analyzing current operations and clarifying the to-be state of the new business (procedures, roles and authority, business rules, and so on) along with the scope to be systematized.
ExplanationIn the software life cycle (the flow from planning a system through development and operation), this work belongs to the requirements definition process (要件定義プロセス) that follows planning. Its aim is to make the scope of systematization and the functions needed there concrete, and to align understanding with stakeholders (the people and organizations involved with the system, such as the departments that will use it and the party placing the order).
AnalogyIt is like listing what is inconvenient about your current home before moving, then agreeing as a family on how you will live in the new one. Drawing the floor plan comes later.