Pupil

The pupil is the variable-sized, black circular opening in the center of the iris that regulates the amount of light that enters the eye. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye.

The pupil, oddly enough, is not even a structure. It's merely a hole in the center of the colored part of your eye (the iris). Via the muscles in the iris, the pupil controls the amount of light entering the eye by getting smaller (constricting) in bright light, and getting larger (dilating) in dim light.