An oculus is a circular opening in a roof which allows for rain and sunlight to spill through the roof of a building. Vatican architecture, especially in Saint Peter's Basilica, makes great use of this architectural device. It is a relic of Roman times, in a way, and was used in circular, vaulted temples like the Pantheon under the Republic.
- Oculus Websters Dictionary. Accessed 12 January 2008.