User script
From GreaseSpot Wiki
User scripts, or userscripts, are scripts that make on-the-fly changes to specific web pages on the client side (in the browser or a proxy server), typically to change their appearance or to add or modify functionality.
User scripts for Greasemonkey and Greasemonkey-compatible alternatives are written in JavaScript, but there is at least one example of using another scripting language, namely Ruby in MouseHole.
For tools other than Greasemonkey that apply user scripts to web sites, see Cross-browser Userscripting and Wikipedia.
