nystagmus vs eye movement

nystagmus and eye movement both are nouns.

Word	Noun	Adjective	Verb	Adverb
nystagmus	Yes	No	No	No
eye movement	Yes	No	No	No

As nouns, eye movement is a hypernym of nystagmus; that is, eye movement is a word with a broader meaning than nystagmus:

nystagmus: involuntary movements of the eyeballs; its presence or absence is used to diagnose a variety of neurological and visual disorders
eye movement: the movement of the eyes

nystagmus (noun)	eye movement (noun)
involuntary movements of the eyeballs; its presence or absence is used to diagnose a variety of neurological and visual disorders	the movement of the eyes