bull’s eye
A round or oval panel or aperture. In the latter case, it may be glazed, open, or louvered. A bull’s eye is often encircled by a double arched frame with voussoirs. A rondel (or roundel) is a small bull’s eye or circular panel; in glazing it looks something like the bottom of a bottle. (Phillips, 1994)
oculus, rondel, roundel