miosis /mī·ō″sis/ [Gk, meiosis, becoming less] , 1. contraction of the sphincter muscle of the iris, causing the pupil to become smaller. Certain drugs and stimulation of the pupillary light reflex result in miosis. 2. an abnormal condition characterized by excessive constriction of the sphincter muscle of the iris, resulting in pinpoint pupils. Compare mydriasis.
