a piece of text or an image that has been reduced in size to fit onto a very small circular object

Today's spies are as likely to hide codes on an iPad as in a microdot. 当今的间谍把代码藏在iPad的可能和藏在微点里一样大。

Microdot cameras produce images so small that they can be hidden in the full stop of this sentence. 微点相机可以把图片制得极小，可以藏在标点句号里。