Add-on and Plug-in

Add-on noun - Something that is not necessary in itself but adds to the convenience or performance of the main piece of equipment.

Plug-in is a synonym for add-on. In some cases you can use "Plug-in" instead a noun phrase "Add-on".

Plug-in noun - A printed circuit that can be inserted into expansion slots in a computer to increase the computer's capabilities.

Add-on is a synonym for plug-in. Sometimes you can use "Add-on" instead a noun phrase "Plug-in".