XSL (Extensible Stylesheet Language)

A language used to transform XML-based data into HTML or other presentation formats, for display in a Web browser.

The transformation of XML into formats, such as HTML, is done in a declarative way, often making it easier and more accessible than through scripting. In addition, XSL uses XML as its syntax, freeing XML authors from having to learn another markup language.

[Taken from the MSDN XML Glossary.]