Web Browser Question:

In web browsers, a plug-in (also called plugin, addin, add-in, addon, add-on, snap-in or snapin) consists of a computer program that interacts with a host application (a web browser or an email client, for example) to provide a certain, usually very specific, function "on demand". Add-on is often considered the general term comprising plug-ins, extensions, and themes as subcategories.