What defines miosis in terms of pupil size?

Miosis is defined as an abnormal constriction of the pupil, specifically when the pupil size is less than 2 mm. This condition can occur in various situations, including exposure to bright light, certain medications, or as a result of neurological conditions. The reduced diameter of the pupil is a protective mechanism to limit the amount of light entering the eye and can also occur in response to opioids or some other drugs that affect the nervous system.

The other size definitions provided do not characterize miosis. Pupils greater than 6 mm would indicate mydriasis, or dilation, while a pupil equal to 3 mm or variable size does not signify any specific abnormality related to pupil constriction. Therefore, miosis is distinctly recognized by its measurement of less than 2 mm, making this definition essential for understanding this condition in clinical settings.