Nystagmus

Short, jerking movements of the eye which are very rapidly repeated and always occur in the same direction. The movements of the eye, as a whole, are not affected by it. Defective vision of such cases is not to be at-tributed to the nystagmus, but, on the contrary, is the cause of it. Vertical n., the eyescontinually move vertically. Lateral n., the eyesconstantly move horizontally. Rotary n., the eyes constantly rotate.

Source: Ophthalmic Dictionary including pronunciation, derivation and definition of the words used in Optometry and Ophthalmology by James J. Lewis, Oph. D. Professor of Optometry in the Northern Illinois College of Ophthalmology and Otology, Chicago.