Software Plug-in (computing), a piece of software which enhances another software application and usually cannot be run independently. Browser extension, which modifies the interface and/or behavior of web browsers Add-on (Mozilla), a piece of software that enhances and customizes Mozilla-based applications Expansion pack, an add-on for a video game Hardware Peripheral, an optional computer hardware component that supplements or enhances the functionality of the original unit Video game accessory, a piece of hardware used in conjunction with a video game console for playing video games Other An extension to a house Addon, a person named in the Hebrew bible An adjuvant therapy, an add-on therapy to existing treatment See also Plug-in (disambiguation)