Software that is installed into an existing application in order to enhance its capability. For example, plug-ins are widely used in image editing programs such as Photoshop to add some special effect. Plug-ins are added to Web browsers to enable them to support different types of content (audio, video, etc.). The term is widely used for software, but can also refer to a plug-in module for hardware.
Plug-In, Add-On, Extension
Although the terms "plug-in," "add-on" and "extension" are used synonymously, a plug-in implies adding new functionality, whereas add-on and extension imply more of a modification to an existing function. These distinctions are not evident in the literature however. See plug-in free
and browser plug-in