Etymology

Latin illudere: to mock, to play against. English kept the mockery. THE FIELD uses a narrower office: a demonstration that seeing is an act, compared with a ruler, a photometer, or a second look.1

Semantic range

Ambiguity (Necker, duck–rabbit). Fiction of contour (Kanizsa). Distortion of length (Müller-Lyer). Brightness that uses a surround (simultaneous contrast, Adelson). Physical effects such as a stick in water are optics, not this office.

What the gloss loses

Illusion as ‘your eyes lied’ misses that the visual system is often doing useful work (constancy, completion, depth).

Misuse

A carnival of tricks. A proof that nothing is real. A spiritual message in a spinning dancer.