Miosis is a medical term for constriction of the pupil. It is the opposite of mydriasis. It is seen in a variety of conditions; important conditions are:

• Horner syndrome (a set of abnormalities in the nervous supply of the face due to damage to the sympathetic nervous system).
• Pancoast tumor (a tumor of the apical lung), due to damage to the ascending sympathetic tract that would normally cause the pupil to dilate.

Miosis should not be confused with meiosis, the cellular division process at the heart of sexual reproduction.