The pupil is an opening located in the center of the iris of the eye that allows light to enter the retina.Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture… (More on Pupil)