What is "geshi"

GeSHi or Generic Syntax Highlighter is a free software library that allows syntax highlighting of source code for several markup and programming languages. The program is written in PHP and is bundled or available as an add-on in popular web-based applications, such as Dokuwiki, Drupal, Mambo, MediaWiki (see extension), phpBB and WikkaWiki. Wikipedia used GeSHi as its primary syntax highlighter until 2015.