Pupillary response is a physiological response that varies the size of the pupils.

A constriction response is the narrowing of the pupils.

A dilation response is the widening of the pupils.

The responses can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light - in low light conditions a dilated pupil lets more light into the eyes - or it may indicate interest in the subject of attention or arousal, sexual stimulation, uncertainty, decision conflict, errors or increasing cognitive load or demand.

Anisocoria is the condition of one pupil being more dilated than the other.

Pupillary response - From Wikipedia, the free encyclopedia