eXtensible Stylesheet Language.

A possible alternate spelling would be "XML Stylesheet Language".

See http://www.w3.org/TR/xsl/

The original intended use was that you would use XSL by using XSLT to convert your XML document into XSL-FO which would then be renderable into printed form by a suitable application. But XSLT has turned out to be a lot more general than that.