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1. A glazed round aperture, glazed with thickened concentric circles of glass; same as glazed bull’s-eye, 2; also called an oculus, oxeye window, or oeil-de-boeuf.
2. An aperture similar to 1., but unglazed; may be open or louvered.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.