Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML (including XML dialects such as SVG or XHTML).

CSS defines how elements should be rendered on screen, on paper, or in other media. It allows web developers to separate content from design, giving more control over layout, colors, fonts, and other visual aspects of web pages.

CSS can be applied to individual elements inline, included in the head of an HTML document, or stored in an external file. Its cascading nature means that styles can be defined globally and then overridden in specific instances, providing flexibility and efficiency in web design.