A condition in which there is involuntary movement affecting the eyes. This movement is mainly horizontal, but can also be rotatory or vertical. In almost every case, both eyes move simultaneously.
In the most widespread type, jerky nystagmus, the eyes constantly move slowly in one direction then quickly in the other direction. This can also arise as a common effect of attempting to follow a sequence of fast moving objects passing the eyes, like when you are looking out of a window of a moving vehicle. This phenomenon is known as ‘optokinetic nystagmus’. More rarely, nystagmus is ‘pendular’, with the eyes moving evenly up and down from one side to another.