Add-on and Adorn

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

Adorn and add-on are semantically related. In some cases you can use "Adorn" instead a verb phrase "Add-on".

Adorn verb - To make more attractive by adding something that is beautiful or becoming.

Add-on and adorn are semantically related. Sometimes you can replace phrase "Add-on" with "Adorn", this terms are similar.