1. （尤指计算机的）附加装置，外挂，插件
An add-on is an extra piece of equipment, especially computer equipment, that can be added to a larger one which you already own in order to improve its performance or its usefulness.
1. a supplementary component that improves capability
Synonym: accessoryappurtenancesupplement
2. a component that is added to something to improve it
e.g. the addition of a bathroom was a major improvement
the addition of cinnamon improved the flavor