Something that plugs in

1. A small software program that plugs into a larger application to provide added functionality.
2. A software component that plugs into the Netscape Navigator. Plug-ins permit the Web browser to access and execute files embedded in HTML documents that are in formats the browser normally would not recognize, such as many animation, video, and audio files. Most plug-ins are developed by software companies who have proprietary software in which the embedded files are created. Compare helper application.
3. A small piece of software that supplements a larger program (as a browser)