I. noun
Date: 1946
: something added on: as
a. : a sum or amount added on
b. : something (as an accessory or added feature) that enhances the thing it is added to
computer hardware add-on s
II. ˈa-ˌdän, -ˌdȯn adjective
Date: 1955
1. : being or able to be added on
an add-on device
2. : able to be added to
add-on certificates of deposit