The project scope statement refers specifically to the input output mechanism the serves the purpose of providing a narrative description of the entirety of the scope for the respective project. This narrative description should include any and all semblances of major deliverables, any and all project assumptions, any project objectives that have been determined, any project constraints, and a statement of work that has been drawn up by the project management team and or the project management team leader. This narrative ultimately provides the basis for making decisions in regards to the project in a future date in time, as well as to make sure that all of the relevant stakeholders in the process are fully abreast and apprised of the project scope that has been worked on to date, meaning the exact determination to date of, basically, what needs to be worked on and in what manner it should be worked on.
This term is defined in the 3rd and the 4th edition of the PMBOK.