optical illusion

This term describes a specific failure of the visual system where the brain perceives something that is not actually present or is distorted in shape, color, or size. It is distinct from a hallucination, as an optical illusion is triggered by an external physical stimulus that is misinterpreted, whereas a hallucination occurs without any external stimulus.

In technical or scientific contexts, it is often categorized into physical illusions, caused by light refraction or reflection, and physiological illusions, caused by the way the eye and brain process signals. In casual conversation, it is frequently used to describe surprising visual tricks or "mind-bending" art.