Final answer:
The round opening in the dome of the Pantheon is called an oculus, and it is a key architectural and symbolic feature of this ancient building.
Step-by-step explanation:
The round opening in the dome of the Pantheon is called an oculus. The Pantheon's oculus is a unique architectural feature, serving as the only source of natural light for this ancient Roman temple. The dome itself is a majestic architectural wonder, constructed from unreinforced concrete, and remains the largest of its kind in the world. The oculus not only illuminates the interior but was also significant in the building's symbolism and may have been used to mark solar and lunar events, reflecting the Roman conception of the dome as a canopy of the heavens.