style sheet

Two words. Use to refer to a file of instructions for formatting a document or to a list of words and phrases and how they're used or spelled in a particular document.

In XML context, use style sheet to refer to an .xsl file. Use Extensible Stylesheet Language (or its abbreviation, XSL, if the term is familiar to your audience) to refer to the related technology.

See also Cascading Style Sheets (CSS)