In web development, CSS stands for Cascading Style Sheets. It is a stylesheet language used to describe the presentation of a document written in HTML or XML. CSS controls the visual appearance of web pages, including layout, colors, fonts, and overall design. By separating content from design, CSS allows developers to maintain and modify styles without altering the underlying HTML structure. This flexibility enhances website functionality and accessibility while enabling responsive design practices for various devices. Overall, CSS is essential for creating visually appealing and user-friendly web interfaces.