add-on noun 4add on verb 2forms 5 noun4 •[before a noun] Something which can be appended to something else.Payments made before an add-on sale must be allocated entirely to the previous contract or contracts. — 1969 •A software extension or hardware peripheral that provides additional functions or customization for a core application or system.Microdrives that can hold several gigabytes (see Figure 2.20) have been available as add-ons for portable devices. — 2010 •Additional chips that can be purchased at specific times in tournament play. •Synonym of expansion pack. add on verb2 •To incorporate (something extra, often an amount of money) into something else (often a bill). •To put an expansion (addition) onto a building, usually a home. formsadded on · adding on · adds on · add ons · add-ons