1.addition的基本意思是“加”,指将两个或两个以上的人或事物合在一起,作此解时为不可数名词,但有时其前可加不定冠词,作“加法”解。
2.addition也可指“增加的人〔事物〕”,指在原有的基础上增加的人或物,此时为可数名词。

in addition (to)
另外,加之,除…之外(还)as an extra person, thing or circumstance
You do that in addition?这是你额外的工作吗?
In addition to giving a general introduction to computers, the course also provides practical experience.课程除了介绍一般电脑知识外,还提供实际操作的机会。

The addition of flour will thicken gravy.加了面粉肉汁会变稠。
That is a simple addition.那是简单的加法。
We've just had an addition to the family.我们家里刚刚添了一口人。
Such an outfit would be a useful addition to my wardrobe.在我的服装中添置这一套服装会有用的。