Clearly defining the scope of a project can be a challenge. The scope definition establishes the value that the project will deliver. Some of the tools include the scope statement, the work breakdown structure, and your change management system.
A solid WBS is the key tool for the entire project plan - and even turn the project execution, a WBS at the right level of detail can help you negotiate scope trade-offs. It's the one thing that no good project should be without.