add-on

名词 add-on:

1. a component that is added to something to improve it
   同义词：addition, improver
2. a supplementary component that improves capability
   同义词：accessory, appurtenance, supplement

动词 add-on:

1. make an addition
   同义词：add on
2. add to the very end
   同义词：append, add on, supplement, affix