What is the X in X-mas?
medical Latin, from Greek nystagmos "nodding, drowsiness," from nystazein "to nod, be sleepy," from PIE *sneud(h)- "to be sleepy."
nystagmus nys·tag·mus (nĭ-stāg'məs)
A rapid, involuntary oscillatory motion of the eyeball.
involuntary back and forth, up and down, or circular movements of the eyes that are often described by observers as "jumping" or "dancing" eye movements. One type of nystagmus, called pendular nystagmus, is characterized by even, smooth eye movements, whereas in the type referred to as jerk nystagmus the movements are sharper and quicker in one direction than in the other. Jerk nystagmus can occur normally, such as when one is dizzy (e.g., from spinning around in circles) or is watching objects pass by quickly from the window of a moving vehicle. Pathologic nystagmus may be present at or shortly after birth because of retinal or optic nerve abnormalities, cataracts, albinism, or a host of other conditions (sensory nystagmus). Alternatively, people can be born with nystagmus and no associated abnormalities of the eye (congenital motor nystagmus). Often there is a gaze or a head position that the affected individual adopts in which the nystagmus is least severe and visual acuity is optimized (called the null point)