In computing, a plug-in is a set of software components that adds specific capabilities to a larger software application. If supported, plug-ins enable customizing the functionality of an application. For example, plug-ins are commonly used in web browsers to play video, scan for viruses, and display new file types. Well-known plug-ins examples are the Adobe Flash Player, Adobe Acrobat, QuickTime and Microsoft Office 2007.
» http://en.wikipedia.org/wiki/Plugin