Statement of intent
A document laying out the project behind the work: why this book, why you, why now, for whom.
The statement of intent accompanies a project submitted to a publisher, producer or grant committee: it lays out not the story (the synopsis's job) but the project — why this book, why you to write it, what angle, what audience, what place in the existing landscape. It's a document of conviction.
Its structuring questions: what makes this text necessary (urgency)? What authorizes or binds you to it (legitimacy — lived experience, research, obsession)? How does the treatment differ from what exists (singularity)? Required for screenplays, essays and grant applications, it's also a clarity exercise for yourself: drafting it sometimes reveals you don't yet know why you're writing this book — precious information before 300 pages.
Example
"This novel grows from one question: what remains of a language when its last speaker dies?"