What is "oeil-de-boeuf"

OEil-de-boeuf \[OE]il`-de-b[oe]uf"\, n.; pl. [OE]ils-de-b[oe]uf. [F., lit., eye of an ox.] (Arch.) A circular or oval window; -- generally used of architecture of the 17th and 18th centuries. A famous room in the palace of Versailles bears this name, from the oval window opening into it.

Oeil-de-boeuf, also œil de bœuf, ( French, "bull's eye"), and sometimes anglicized as ox-eye window, is a term applied to a relatively small oval window, typically for an upper storey, and sometimes set on a roof slope as a dormer, or above a door to give light. Windows of this type are commonly found in the grand architecture of Baroque France. The term is also so often applied to similar round windows, like those found in Georgian architecture in Great Britain, and later Greek Revival and Colonial Revival styles in North America, that this must be considered part of the usage. The term initially applied to horizontal oval windows, but is also used for vertical ones.