The Collaborative International Dictionary of English v.0.48:

oeil-de-boeuf \oeil`-de-boeuf"\, n.; pl. oeils-de-boeuf. [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.
   [Webster 1913 Suppl.]