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1. Make an addition
   "Let's add on to this"
    
2. Add to the very end
   "He added on a glossary to his novel where he used an invented language";
   - append, supplement, affix

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

Derived forms: added on, adding on, add-ons, adds on

Type of: add, attach, component, computer software, constituent, element, package, software, software package, software program, software system

Encyclopedia: Add-on