An Israeli company called ID-U Biometrics thinks that the patterns of eyeball movement are unique to each individual, and are therefore a definitive means of identifying people:

In ID-U Biometrics’ system, the user has to watch a moving object onscreen, while the camera observes the motion of their eyes. Since the way our eyes move is based on a combination of factors –such as anatomy, physiology, behavioral characteristics, eye structure–it’s a signature that simply can’t be duplicated or forged, according to its developers.[…]

This approach differs radically from eye-related biometrics we’ve written about previously, such as iris scanning. Iris scanning systems rely on matching the image of your iris structure with a stored pattern of your iris. In contrast, the pattern the ID-U technology is based on consists of dynamic movements made by your eyes as they track a target, something that cannot be controlled or learned. “Most of the eye movement components are involuntary, and we are not aware of them at all,” says Palti-Wasserman.