(1) SYN: fenestra. (2) Any opening in space or time, particularly a critical interval within which a given event must, or cannot, occur. (3) computed tomography the range of CT numbers (in Hounsfield units) across which all shades of the gray scale are distributed in a given image so as to emphasize slight differences in x-ray absorption coefficients between tissues of similar density (e.g., mediastinal soft tissues).CT number, window level, window width.