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Add-ons are optional computer hardware or software modules that supplement or enhance the original unit they are adding on to. Also known as plugins, extensions, snap-ins, or various other vendor-specific names, these modules often have proprietary interfaces that are tightly held by the manufacturer in order to prevent competing companies from producing add-ons. Manufacturers can use add-ons to create vendor lock-in by limiting upgrade options to only those available from or endorsed by the original manufacturer. IBM's Micro Channel architecture, technically superior to Industry Standard Architecture as a way to add components to IBM PCs, largely failed to gain wide support due to the difficulty in getting certification for third-party devices.^{[citation needed]}

Also known as a peripheral or mod, a hardware add-on is a device generally sold separately from the main unit, but which connects to the main unit to add significant new functionality. Hardware add-ons are probably most commonly associated with computers and video game consoles, but there are other uses for the term. A hardware add-on differs from an accessory in that an accessory may add less significant functionality, or in some cases may only add aesthetic value. It differs from a hardware component in that it is not required for normal, essential operation of the main unit. For example, a network switch may be equipped with an unoccupied but non-standard port to accommodate various optional physical layer connectors.

See also: Video game accessory

Games and productivity applications often use plugin architectures which allow original and third-party publishers to add functionality. The Microsoft Flight Simulator series is famous for its downloadable aircraft add-ons.^{[citation needed]} Web browsers use plugins to enable the presentation of new content formats without modifying the underlying web browser. The Mozilla Foundation defines add-on, in reference to Firefox and related software, as an inclusive term for a category of augmentation modules that are subdivided into plugins, extensions, themes, and search engines.^[1]

There are two known music groups or artists with "Add-on" in their band names. The Add+Ons,^[2] from the United States, and The Add-Ons,^[3] from the United Kingdom.

Look up add-on in Wiktionary, the free dictionary.

1. ^ Mozilla Add-ons.
2. ^ The Add+Ons' MySpace profile.
3. ^ The Add-Ons' MySpace profile.