1. A book in which are recorded the debts and credits, or accounts of the day.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• say book,
• xay book,
• cay book,
• fay book,
• ray book,
• eay book,
• dzy book,
• dsy book,
• dwy book,
• dqy book,
• dat book,
• dag book,
• dah book,
• dau book,
• da7 book,
• da6 book,
• day vook,
• day nook,
• day hook,
• day gook

The adjective form of the word "day book" is "diurnal".

The plural form of "day book" is "day books".

The singular form of the word "day book" is "day book" itself.