Sass (Syntactically Awesome Style Sheets) is a stylesheet language that is compiled to CSS, the language that sets the colours, fonts and layout of web pages. A compiler is a program that translates source code into another language; here a Sass file becomes a plain CSS file that any browser can read. Sass is made by the Sass team and many contributors, and it has been supported since 2006.
Sass adds features that plain CSS lacks: variables, nested rules, mixins (reusable groups of declarations), functions, control flow such as @if and @each, and modules loaded with @use. It has two syntaxes. SCSS, with the extension .scss, is a superset of CSS, so almost all valid CSS is valid SCSS. The older indented syntax, with the extension .sass, uses indentation instead of braces. The current implementation is Dart Sass, which runs from the command line and is also published on npm as a pure JavaScript package. Frameworks such as Bootstrap are built with Sass.
Sass is free and open source under the MIT license.