Cascading Style Sheets (CSS) is a stylesheet language used to describe the look and formatting of a document written in a markup language. CSS is used to define the layout and appearance of web pages written in HTML or XHTML, and can be used to style other types of documents as well. It is a way to separate the content of a document from its presentation, allowing users to apply different styles to a single document, or to multiple documents at once. CSS allows developers to create consistent and reusable styles, which can make it easier to maintain and update web pages over time.