add-on add-on /ˈædˌɒn/

noun

1. A supplementary component that improves capability  ( add-on , appurtenance , bolt-on , accessory , supplement )
   a supplementary component that improves capability
   
2. A component that is added to something to improve it  ( add-on , improver , addition )
   a component that is added to something to improve it
    The addition of a bathroom was a major improvement
   the addition of a bathroom was a major improvement
    The addition of cinnamon improved the flavor
   the addition of cinnamon improved the flavor
   
3. A software module that extends or enhances the capabilities of an existing application  ( add-on , plug-in )
   a software module that extends or enhances the capabilities of an existing application