Start by understanding why you are doing something, then define what it is. Not the other way around.
Good projects start with the end result in mind.
Don't just think what your project is write it down. Forcing yourself to write it down is a great way to ensure it makes sense.
Success in developing a Project Definition requires that the information be specific, precise, complete, unambiguous and concise.
Make sure you know if you are responsible for 'what' the project delivers or also for ensuring it achieves the 'why'. It is easier just to be responsible for the 'what', and this is normally all a project manager is responsible for.
If you are running a project for someone else, ensure they understand the Project Definition and the implications of its contents, and get them to sign it.