APA Dictionary of Psychology

a condition characterized by partial ptosis (drooping of the upper eyelid), miosis (excessive constriction of the pupil), and often anhidrosis (absence of sweating) on the same half of the face. Congenital or acquired, it is caused by damage to the pons region of the brainstem. There may be lack of coordination in eye movements (saccades and smooth-pursuit eye movements). Also called oculosympathetic paralysis. [Johann Horner]