1. In medicine, the period in the course of a disease when the symptoms begin to diminish.
2. A contraction of the pupil of the eye; such as, a contraction caused by a reaction to a drug.
3. Constriction of the pupil of the eye, resulting from a normal response to an increase in light or caused by certain drugs or pathological conditions.

Miosis occurs in certain fevers, congestion of iris, typhus, early stages of meningitis, some forms of drug poisoning, brain lesions, and sunstroke.