BRDs, SRSs, user stories, use cases and traceability matrices.
Business Requirements Document
What the business needs and why, stated without prescribing a solution.
Software Requirements Specification
System-level functional and non-functional requirements, each one testable.
User Story
A small unit of value with acceptance criteria you can argue about before the work starts.
Use Case Specification
Step-by-step actor–system interaction, including every way it can go wrong.
Requirements Traceability Matrix
One table proving every requirement reached design, code and test — and that nothing untraceable was built.