The round opening in the dome of the pantheon is called what?

In the architectural context, the oculus is a small hole or circular window, built with the intention of entering light, air in circulation or simply for decorative purposes.

One of the oldest oculus is the one located at the Roman Pantheon (temple), a construction that began in the 27th century BC on the orders of Marcus Agrippa for Augustus and was completed by order of the Emperor Hadrian.

because if the tool that you are using is not balanced it there would be false mass.