Illustrated Architecture Dictionary
A round aperture, open, louvered, or glazed
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)
Examples from Buffalo architecture: