One picture, two answers

An illusion is a misrepresentation of a “real” sensory stimulus: an interpretation that contradicts objective “reality” as defined by general agreement. Britannica Scholars: illusion

An illusion is distinguished from a hallucination, which seems to originate without an external source of stimulation. Britannica Scholars: illusion Neither an illusion nor a hallucination is necessarily a sign of psychiatric disturbance, and both are regularly reported by virtually everyone. Britannica Scholars: illusion

Many common visual illusions are perceptual: they result from the brain’s processing of ambiguous or unusual visual information. Britannica Scholars: illusion

Perception, in humans, is the process whereby sensory stimulation is translated into organized experience. Britannica Scholars: perception In the language of Gestalt psychologists, immediate human experience is of organized wholes, not of collections of elements. Britannica Scholars: perception

On first looking at a drawing of this sort, you will probably immediately see a meaningful picture. Britannica Scholars: perception After continued gazing, the initial percept may abruptly be replaced by another. Britannica Scholars: perception

Thereafter, the two percepts should alternate with the passage of time. Britannica Scholars: perception

This site looks at three well-known cases: the vase-and-faces figure, the Necker cube, and the hollow face.