N-COUNT
（尤指計算機的）附加裝置，外掛，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...
揚聲器作為隨身聽的附加裝置出售。
Nintendo hopes to price the add-on modem at less than $200.
任天堂希望將該外掛調制解調器的價格定在200美元以下。