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. "Extensions" are plug-ins added to Web browsers to enhance their capabilities. The plug-in term is widely used for software but can also refer to a hardware module that plugs into a system. The terms "plug-in," "add-on" and "extension" are used synonymously. See
plug-in free and
browser extension.