A style guide (or manual of style) is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization, or field. (It is often called a style sheet, though that term has other meanings.)

Website Style Guide
Great UI Design Requires Smart Style GuidesUX PinMarch 27th, 2017
Front-End Style-Guides: Definition, Requirements, Component ChecklistNielsen Norman GroupMarch 27th, 2016
The Elements of a Style GuideUX MagazineSeptember 24th, 2013