Brandbook
A comprehensive document defining a project's visual identity, typography, color palette, and guidelines for consistent application.
In practice
A brandbook is the reference manual for a visual identity: how to use the logo, which colors and typefaces to use, spacing and sizing rules, image style, tone of voice, and examples of correct and incorrect use. It lets designers, printers and agencies who never met the original team produce consistent work. A good brandbook shows real applications, such as a sign or an email header, not just abstract rules.
Often confused with Visual Identity
The visual identity is the system itself; the brandbook is the document that records and explains how to use that system.
Try this
Write a one-page brandbook for a fictional bakery: logo rules, two colors, one typeface and one example of misuse.