From HomoExcelsior

1. A small, circular or crescent-shaped opening in a vaulted roof. 2. A crescent-shaped or semicircular space, usually over a door or window, that may contain another window, a sculpture, or a mural. 3. A fortification that has two projecting faces and two parallel flanks. 4. A broad, low-lying, typically crescent-shaped mound of sandy or loamy matter that is formed by the wind, especially along the windward side of a lake basin.

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