Plugin

Many software applications have been developed with the ability for outside developers to add their own extra features to the main software. This extra software is called a 'plugin'.

For example, Photoshop has a thriving community of plug-in developers. They make fancy filter effects, image adjustment tools and so on.

The web browser 'Firefox' also has many people developing useful plug-ins that improve on what the basic Firefox application can do. In this case the plug-ins are called add-ons (see https://addons.mozilla.org/en-US/firefox/)

Some browser plug-ins are so useful that most people need to install them if they are to get the best out of the web - for example the 'Flash Player' plugin from Adobe is needed to view Flash movies.