Art bible
Also said as Style guide, Visual bible
A shared document that sets how the game looks and how to get that look: palette, shapes, materials, proportions and line, with examples of what is right and wrong.
A large team of good artists will drift apart, each one drawing their own idea of the game. A bible lets dozens of people, and outside studios, make things that look as if one hand made them.
Say it
“Write the art bible before outsourcing: a palette of 12 colours, how thick lines get, corner shapes, three examples each of right and wrong, and how scale compares to the player.”
Not to be confused with concept art
Concept art imagines what a thing might look like. An art bible states the rules every finished thing has to follow.