written proposal; project plan document
A formal document outlining a plan, project, or idea, commonly used in business contexts.
来週の会議までに企画書を提出してください。
Please submit the project proposal by next week's meeting.
彼は新しい商品の企画書を書いている。
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').