Iris: The iris is the colored part of your eye that surrounds and controls the size of your pupil.
Dilate: To dilate means to expand or enlarge. In terms of the pupil, dilation refers to when it becomes larger to allow more light into the eye.
Constrict: To constrict means to narrow or shrink. In terms of the pupil, constriction refers to when it becomes smaller to limit the amount of light entering the eye.