Illustrated Architecture Dictionary.

Oculus
Plural: oculi
Pronunciations: AHK you lis; AHK you lie

A round opening or window without stone tracery (especially found in Romanesque churches)

A small circular opening, and which was a precursor of the Gothic rose window

Window that has an oval or circular shape (as of an eye)

Etymology: Latin, "eye"

Name of the round opening in the top of the dome of the Pantheon in Rome

An oculus, circular window, or rain-hole is a feature of Classical architecture since the 16th century. They are often denoted by their French name, oeil de boeuf, or "bull's-eye". Circular windows set in dormers have been a feature of French Classical architecture since the beginning of the seventeenth century. For structural reasons, they are also found as the portholes of ships. - Wikipedia (Nov. 2011)

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Examples:

• Illustration above: Pantheon, Rome, Italy
  
  
• Forum Thermal Baths, Pompeii, Italy
  

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