An oculus (; ) is a circular opening in the center of a dome or in a wall. Originating in classical architecture, it is a feature of Byzantine and Neoclassical architecture.
A horizontal oculus in the center of a dome is also called opaion (; ).
Windows of this type are commonly found in the grand architecture of Baroque France. The term is also applied to similar round windows, such as those found in Georgian architecture in Great Britain, and later Greek Revival and Colonial Revival styles in North America, so that must be considered part of the usage. The term initially applied to horizontal elliptical windows, but is also used for vertical ones.Burden, Ernest E., Illustrated dictionary of architecture, McGraw-Hill Professional, 2001, p.354, , Google Books
The spread is not limited to ecclesiastical architecture. This type of window can also be found in the late Romanesque period in the area of secular architecture in the castles of Frederick II of Hohenstaufen, Holy Roman Emperor from 1220 to 1250, (Castel del Monte, Palazzo San Gervasio, on the donjon in the castle of Lucera, etc.), later also in Renaissance palaces and villas and in the Baroque.
2024 Pantheon Rome 04 2024 7134.jpg|The Ancient Roman oculus (opaion) of the Pantheon, Rome, Italy
Image:Plafond hasht behesht esfahan.jpg|Islamic oculus (opaion) opening into a cupola in the Hasht Behesht, Isfahan, Iran
Oculi - - Altar of Our Lady of Sweat - Duomo - Ravenna 2016.jpg|Baroque oculus (opaion) in a dome of the Ravenna Cathedral, Ravenna, Italy
File:Gassicourt Sainte-Anne 327.JPG|Romanesque oculus of the , Mantes-la-Jolie, France
Cathédrale Notre-Dame de Laon, tour-lanterne 02.jpg|Gothic oculus in the Laon Cathedral, Laon, France
Florence duomo fc10.jpg|Renaissance oculus of the Florence Cathedral, Florence, Italy
Parc de Bagatelle @ Paris (27765814394).jpg|Rococo oculus in the Parc de Bagatelle (Paris)
File:South gate of the Petit Trianon 004.JPG|Louis XVI round window of the Petit Trianon (Versailles, France), with a festoon-derived ornament at the top
Georgsplatz 20, Fensterdetail, Hannover.jpg|Gothic Revival oculus of the Hannoversche Bank - Haus III on Georgstraße, Hannover, Germany
File:Opéra municipal de Clermont-Ferrand, œil de bœuf.jpg|19th century Eclectic Classicist oculus of the , Clermont-Ferrand, France
File:Fenêtre sur le toit du Palais de Justice.JPG|Beaux-Arts dormer oculus of the Building of Préfecture de Police de Paris, Île de la Cité
Louis Baralis HSBC 1.jpg|Art Nouveau oculus of the , Paris
La Maison Bleue, porte d'entrée - Angers - 20110119.jpg|Art Deco oculus above a door in Angers, France
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