What is an oculus in ancient rome?

Oculus (plural oculi) in Latin means eye. The name referred to circular Windows or openings.

On type of oculus was the stone opening of the impluvium of the houses of the rich and some public buildings. It lied 30 cm below the floor and was the top of an underground holding chamber which collected rainwater from the compluvium, the roofless central part of the atrium (courtyard) and had layers of gravel and sand to filter the water. These two Words come from pluvia, the Latin for rain.

There is a circular opening at the top of the dome of the Pantheon in Rome. Originally this was a temple dedicated to all gods. Then it became a Christian church. The building is circular and its walls support the dome. The oculus is the only source of light. It creates a beam of light that reaches the floor and rotates during the day with the movement of the sun in the sky. It is not covered and the rainwater is cleared by drainage canals.