Illustrated Architecture Dictionary
Bull's-eye window / Oeil-de-boeuf window
Oeil-de-boeuf window, also called bull’s-eye window, in architecture, a small circular or oval window, usually resembling a wheel, with glazing bars (bars framing the panes of glass) as spokes radiating outward from an empty hub, or circular centre.
In French, oeil-de-boeuf means “eye of the steer,” and, in the French chateau of Versailles, erected for Louis XIV between 1661 and 1708, there is a small antechamber called the oeil-de-boeuf room, which is lighted by such a small, round window. This type of window is also frequently featured in the Jacobean manor houses of 17th-century England.
- Encyclopaedia Britannica (online October 2020)
Until modern techniques were developed, one way of making window glass was to spin a blob of molten glass at the end of a tool called a pontil. Centrifugal force flattened the glass into a sheet, and when the tool was pulled away it left a characteristic bull's eye mark.
The relatively flat glass on the perimeter of this glass pancake was carefully cut into pieces and sold to the high-brows who could actually afford windows.
What was left, the stuff with the bull's eyes, went into lesser locations, like barns and sheds.
- Old House Web: Bulls Eye Glass (online May 2017)
Bull's eye corner blocks are found in Federal and Greek Revival styles
Oculus is a broader term: Window that has an oval or circular shape resembling an eye.
Examples from Buffalo: