Oh jeez. Did you son have it when he was looking at something moving, or all the time? I looked nystagmus up, but I don't totally understand all of it.

It sure is a thing, it sounds a lot like Nystagmus (my son, 18, has it and it was detected when he was quite young). I suggest you consult with an ophthalmologist ASAP. Most people have never heard of it, but a few famous people have it, like Kenny Rogers, the singer from the 1960-70s.

http://www.allaboutvision.com/conditions/nystag...

We'll if you call her name and she stopped, probably not a seizure. Those are usually not controllable like that. Possible a tic. Typical with ASD and ADHD diagnosed children, though I have heard some children with ASD have tics. They do them different ways, blinking eyes rapidly is usually seen too. Ask a neurologist or a psychoneurologist if it may be one. Stimming depends on the area, kinda... usually stimming is something done regardless of location. In other words, if its only done in the car, well they're bored, like a person clicking a pen over and over. but if they do it at home too, and at the park, at school, in the tub (see where im going) then its stimming. Maybe a tablet or a book in the backseat, toys. Boredness increases tics, if it is that (like the pen, which is kinda like a tic)

Seizure was the first thing that came to my mind when I saw it. But when I called her name she stopped. But I am going to mention it. Thank you

I suspect it could range from just intense focus (and nothing serious), through stimming, to even seizure activity. I hope it's nothing serious certainly, but I recommend you speak to her developmental pediatrician.
I hope it checks out ok.