False perceptions or beliefs; things that appear to exist but do not, or appear different from reality. Deceptive visual or sensory experiences.
From Latin illusio, from illudere meaning 'to mock, play with', from in- + ludere (to play). The word evolved from meaning 'mockery' to 'deception of the senses' by the 14th century.
Optical illusions reveal the active, interpretive nature of human perception - our brains don't just record reality like cameras, but constantly construct our experienced reality based on expectations, context, and past experience. This makes illusions powerful tools for understanding consciousness itself.
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