microdotnoun [ C ]
/ˈmaɪ.krəʊ.dɒt//-kroʊ.dɑːt/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.
Microdot cameras produce images so small that they can be hidden in the full stop of this sentence.