ICARUS is a proprietary scripting language/format used by Raven Software in their titles which use the Quake 3 engine. The first game to use this was Star Trek Voyager: Elite Forces. Jedi Outcast, Soldier of Fortune II, and Jedi Academy also used this scripting language.
Icarus scripts are initially written as text files. A script is compiled into a binary .ibi file via IBIZE.exe (which is included in the SDK); these are the files that are used by the game engine. A user-friendly interface for ICARUS scripting is present in the form of BehavED.