Plug-in
From The Glossary of Digital Photography
A program or subprogram used in conjunction with the main program that it plugs into. Photoshop, for example, has many plug-ins available to either perform functions that Photoshop itself doesn't do, or to do existing functions better. Sharpening and noise reduction are two areas in which many photographers choose to use a plug-in. Plug-ins for reading special image formats, or for using web cams with browsers, among other functions, are also available..
