noun
written contract; written agreement
A formal document that records the terms of an agreement; used in legal, business, and official contexts.
両社は約定書に署名した。
Both companies signed the written contract.
約定書の内容をよく確認してください。
Please carefully check the contents of the written agreement.
Compound of 約定 (agreement, promise) and 書 (document). The exact historical derivation is uncertain, but the word is conventionally associated with formal written agreements.