Explain the difference between an eye-tracker and eye-drift.

An eye‑tracker is a hardware or software system that records the position and movement of a person's eyes in real time, typically using infrared cameras or electrooculography. It provides quantitative data such as fixation points, saccades, and gaze duration, which are used in HCI research, marketing, and accessibility. Eye‑drift, on the other hand, refers to a natural, involuntary movement of the eye while the person is supposedly fixating on a single point. It is a physiological phenomenon caused by the eye’s muscles and the brain’s control system; it can be measured by an eye‑tracker but is not a device itself. In summary, the eye‑tracker is the tool that captures eye movement data, whereas eye‑drift is one of the specific types of movements that can be observed and analyzed by that tool.