Miosis, or myosis (from Ancient Greek μύειν (múein) 'to close the eyes'), is excessive constriction of the pupil.

Light entering the eye strikes three different photoreceptors in the retina: the familiar rods and cones used in image forming and the more newly discovered photosensitive ganglion cells.

The ganglion cells give information about ambient light levels, and react sluggishly compared to the rods and cones.

Signals from photosensitive ganglion cells have multiple functions including acute suppression of the hormone melatonin, entrainment of the body's circadian rhythms and regulation of the size of the pupil.

Visceromotor nerve axons (which constitute a portion of cranial nerve III, along with the somatomotor portion derived from the Edinger-Westphal nucleus) synapse on ciliary ganglion neurons, whose parasympathetic axons innervate the iris sphincter muscle, producing miosis.