A visual illusion is an image or effect that looks different from the physical steps that create it. In hardware, this can happen when rapid light changes, motion, or perspective make the eye and brain merge separate frames into one stable picture. That is why persistence-of-vision displays can appear to show a complete image even though no single instant contains the whole frame.
In cyber security, visual illusions matter because attackers often rely on what users think they see, not what is actually present. Fake login pages, manipulated screenshots, overlay attacks, and deceptive QR codes can all exploit visual shortcuts and trust in familiar interfaces. Defenders also use controlled visual effects, such as anti-phishing warnings, to make important changes harder to miss. The key lesson is to verify the underlying source, not just the appearance.