The pupil is the hole located in the centre of the iris of the eye that allows light to strike the retina. The black appearance of the pupil is due to light rays entering the pupil either being absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye. The lack of light available to exit the narrow pupil results in the black colour.
In optical or camera terms, the pupil is equivalent to an eye's aperture and the iris is equivalent to the aperture-, or F-stop. The primary function of the pupil is to regulate, or control, the amount of light that enters the eye. In bright environments, the pupil becomes smaller, permitting less light to the enter the eye. Conversely, the pupil becomes larger as light levels decrease to allow more light into the eye. Doctors often examine the pupil to determine one's neurological function.
In humans the pupil is round, but other species have vertical slit pupils, horizontally oriented pupils, or annular pupils.