LACUNAR. A panelled or coffered ceiling; also the sunken panels or coffering in such a ceiling.
From the Penguin Dictionary of Architecture, Third Edition
Them Roman guys was real smart like. That dome is made out of concrete, and each one of those coffers had to be designed and constructed to become smaller in size and volume as they march their way up to the oculus at the top. In order to make the dome lighter, they incorporated small clay jars into the mix to create voids--in effect, the whole thing is something like a big sponge.
A more recent example, both of lacunars and of impressive concrete work would be portions of I.M. Pei's East Building of the National Gallery. Those might look very simple, but the formwork had to be constructed by a team of cabinet makers, due to the complexity of the geometry and the desire for absolute precision in the final product.
So there you go!