CSS (Cascading Style Sheets) is a powerful stylesheet language used in web development to describe the look and formatting of a document written in HTML or XML. It handles the visual side of web pages, allowing developers to design layouts, colors, fonts and transitions.
It's 'cascading' because of its design principle where priority is given to styles in a specific order, creating a cascade of styles. This allows for flexible and complex designs without cluttering the HTML with formatting details, thus, promoting separation of concerns in web design.