Stylesheet
A stylesheet (also called a style sheet) is a set of CSS rules used to control the layout and design of a webpage or document. The term "style sheet" is used in official specifications such as those from the W3C, while "stylesheet" is commonly used in web development and within MDN. Internal stylesheets are placed inside a <style> element inside the <head> of a web document, and external stylesheets are placed inside a separate .css file, which is applied to a document by referencing the file inside a <link> element in the document's head.
External stylesheets are generally preferred because they allow you to control the styling of multiple pages from a single place, rather than having to repeat the CSS across each page.
See also
- CSS Styling basics
- Stylesheets on Wikipedia