The Work breakdown Structure (WBS)
It not only contains a breakdown of the Product Scope in meaningfull Work packages (that make sense to the customer), but also all supporting processes that support the product and its implementation into the business (if that is part of your project). The WBS defines the Project Scope: what is not in the WBS, is out of scope.
Each Work Package should have a short description of what it contains (the WBS dictionary). You can link each Work Package to more detailed functional and technical specification documents, which are elaborated during the project.
Your network plan (of Work Packages) should clearly show how the product will reach its intended maturity.
You can color code each WBS element, depending on the status of the analysis. This gives a single glance overview of the project.