- A software or hardware component added to a computer or program to give extra features or functions.
- A supplement or accessory added to something else.
Mid-20th century: from add + on.
- A browser add-on that blocks ads.
- The game came with several add-ons that expanded its gameplay.
- A new add-on for the flight simulator program.
- The add-on to the car allows it to run on electric power.
- An add-on feature to the phone's camera.