Oculus (oculus)

A small circular opening, often in the top of a coupole (cupola).
The oculus, when installed as a window in a wall, is also called œil-de-bœuf.
One can find three oculi in the façade (facade) of the Saint-Barthélémy in Fleury-la-Montagne.

Saint-Barthélémy - Fleury-la-Montagne