Userscript
A userscript (or user script) is a program, usually written in JavaScript, for modifying web pages to augment browsing. Uses include adding shortcut buttons and keyboard shortcuts, controlling playback speeds, adding features to sites, and enhancing the browsing history.
On desktop browsers such as Firefox, userscripts are enabled by use of a userscript manager browser extension such as Greasemonkey. The Presto-based Opera-supported userscripts (referred to as User JavaScript) are placed in a designated directory. Userscripts are often referred to as Greasemonkey scripts, named after the original userscript manager for Firefox.
On Wikipedia, a user scripts feature is enabled for registered users that allows them to install userscripts to augment editing and viewing of the encyclopedia's pages.