add-onˈadd-on noun [countableC]1MARKETINGa product that is sold to be used with another productYou can have legal insurance as an add-on to a household policy.
2COMPUTING a piece of equipment or software that you connect to a computer, usually in order to improve its performanceWe’ve compiled an extensive list of add-ons to make browsing with Internet Explorer more productive.
—add-on adjectiveadj [only before a noun]add-on software