Māyā

Not "illusion" in the sense of nothing there — closer to appearance, measurement, or the creative power by which the one shows itself as many. The rope taken for a snake: the fear is real, the snake is not, and what is actually there was never absent.

A live disagreement, not a settled doctrine: Śaṅkara and Rāmānuja fight over exactly how unreal the world may be said to be. And the Kabbalah's parallel term is warmer — the levushim, garments, which conceal in order to make an unbearable light approachable.