written proposal; project plan document
A formal document outlining a plan, project, or idea, commonly used in business contexts.
Please submit the project proposal by next week's meeting.
He is writing a proposal for a new product.
企画室 is the planning department (a place/organization), whereas 企画書 is a planning proposal document—easy to mix up because both start with 企画.
Compound of 企画 (kikaku, 'planning') + 書 (sho, 'document').