Add-on, a.k.a. Plug-in is a set of components that enhances the existing, or adds new capabilities to a software application.
A plug-in system is very common, because it lets other developers (and maybe users) make small plug-ins to add new features quite easily.
Add-ons are also a good customization tools, because they let users chose which of the features do they want, and which they don't. It also helps to keep the distributive small.
There are specific tags for certain platforms / applications, like
- Firefox extensions an add-ons use firefox-addon
- Google Apps Script add-ons use google-apps-script-addon
Usually it's better to use the more specific one.
