add-on I. \ˈ ̷ ̷ ˌ ̷ ̷ \adjective 1.: being or able to be added on < add-on accessories > 2.: of or relating to add-ons 3.: that can be added to < an add-on certificate of deposit > II. \¦ ̷ ̷ ¦ ̷ ̷ \noun (-s) : something added on: as a.: a sum or amount added on b.: something (as an accessory or an added feature) that enhances the thing it is added to