الكتاب: هو القرآن المنزل على رسول الله ﷺ، المكتوب في المصاحف، المنقول عنه نقلاً متواتراً بلا شبهة فيه.
The Book is the Qur’an revealed to the Messenger of Allah ﷺ, written in the codices, and transmitted from him through uninterrupted mass transmission free of doubt.
The definition identifies three features together: revelation, preservation in written codices, and mass transmission (tawatur). These describe the particular revealed text whose legal implications Usul al-Fiqh examines.