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

An add-on, which may overlap with the definition of a plug-in, may refer to:

• an optional computer hardware or software component that, if installed, significantly supplements or enhances the functionality of the original unit, such as:
  • Add-on (Mozilla), a software plugin, extension, or theme that enhances and customizes Mozilla-based applications
    • Mozilla Add-ons, the official Mozilla Foundation online repository for add-ons for Mozilla-based applications
  • Peripheral, sometimes informally referred to as an "add-on"
  • Video game accessory, a piece of hardware used in conjunction with a video game console for playing video games

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