From webopedia.com:
(n.) A hardware or software module that adds a specific feature or service to a larger system. The idea is that the new component simply plugs in to the existing system. For example, there are number of plug-ins for the Netscape Navigator browser that enable it to display different types of audio or video messages. Navigator plug-ins are based on MIME file types.

What plug-ins may be safely installed on school computers?
(Click on the icons for a link to the download site.)

Adobe Acrobat Reader
Real Audio Player
Macromedia Shockwave
Ipix Viewer
Apple Quicktime
Java by Sunmicrosystems
Macromedia/Adobe Flash Player