add-on
/ˈædɒn/noun
1- something that has been or can be added to an existing object or arrangement附加物件：
as modifier cars with add-on extras.有附加零部件的汽车。
1.1
- an accessory device designed to increase the capability of a computer or hi-fi system（增加计算机或音响系统功能的）附加设备。