A general term for any software program or utility that expands the functionality of a larger program or computer system, acting as if they were part of that program or system. On the Macintosh, system extensions (in this case, also called INITs) work as part of the Mac operating system, adding such features as the ability to play back video or audio, or read certain types of files. In a program such as Photoshop, extensions (in this case, also called plug-ins) supply an additional set of filters and ways of manipulating images, or allow for scanner software to work from within the program. Extensions are also called additions or annexes.
The term extension is also an abbreviated term for a file extension. See File Extension.