CSL (Citation Style Language)
Also called: Citation Style Language
An open XML format for describing how a citation style formats references, used by Zotero, Mendeley, Paperpile, and this site.
A CSL file is a machine-readable specification of a style: which fields to print, in what order, with what punctuation, and how to handle the awkward cases. Because it is data rather than code, one citation engine can format in thousands of styles without knowing anything about any of them.
The CSL project maintains a repository of more than 10,000 styles, most of them contributed by publishers and journals. Roughly two-thirds are “dependent” styles: files that contain no formatting at all, only a pointer at the independent parent whose rules they share. That is how hundreds of Elsevier journals can each have their own style name while sharing one implementation.
CSL is why a citation generator can be both broad and exact. The formatting is not an approximation of the style guide — it is the style file the reference managers use.