Illusion, or maya, refers to the concept in Hindu philosophy that the material world is an illusion that veils the true reality of Brahman, the ultimate, unchanging reality. This idea suggests that what we perceive through our senses is deceptive, and the distinctions we make in the world are not truly reflective of the underlying unity of existence. Understanding maya is crucial for recognizing the non-dualistic nature of reality as taught in Vedanta.