Glossary » style guide
a reference that establishes the look-and-feel of a user interface by clearly defining the standards and conventions of that user interface. Style guides usually include the principles that guide the design of the interface, graphic layout grids, exact size and spacing of elements in the interface, fonts, colors, interactive behavior, standard text messages (such as error messages), and labeling standards.
Excellent style guides are available for most major operating systems. Specific style guides are often created for individual applications or websites to establish consistency in organization and visual standards, and to assist the user interface designer in communicating the design both to developers and to other designers who may be working on the same project in the future.