**Definition:**
A Cascading Style Sheet (CSS) is a stylesheet language used to describe the presentation of a document written in HTML or XML. It controls the layout, colors, fonts, and overall visual appearance of web pages.

**Key ideas:**
- CSS allows for separation of content and design, making web development easier.
- It uses selectors to apply styles to specific HTML elements.
- CSS can be applied inline, internally within a document, or externally through linked files.
- It supports responsive design, enabling websites to look good on different devices.

**Why this topic matters in this category:**
Understanding CSS is essential for creating visually appealing and user-friendly websites in computer science.