Miosis is the process of the pupil in the eye becoming smaller or constricting. This reaction occurs in response to bright light or certain medications, helping to protect the retina from excessive light exposure. It is controlled by the iris, the colored part of the eye, which adjusts the size of the pupil to regulate the amount of light entering. Miosis can also be a sign of various medical conditions, such as narcotic use or neurological disorders. In contrast, the opposite process, known as mydriasis, involves the pupil dilating or enlarging. Both processes are essential for proper vision and eye health.