N-COUNT
（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.
例句
Speakers are sold as add-ons for personal stereos...
扬声器作为随身听的附加装置出售。
Nintendo hopes to price the add-on modem at less than $200.
任天堂希望将该外挂调制解调器的价格定在200美元以下。