noun
written request; application form
A formal document used to request or apply for something, such as a service, estimate, or permission. Common in business and official contexts.
見積もりを取るために、取引先に依頼書を送った。
I sent a written request to the business partner to get an estimate.
参加希望の方は、所定の依頼書にご記入ください。
Those wishing to participate, please fill out the prescribed application form.
依頼 is the general act of requesting or entrusting, whereas 依頼書 is the physical or digital document that formalizes that request.
Compound of 依頼 (request, entrust) and 書 (document, writing). The word follows a common pattern for naming formal documents in Japanese.