An illusion is something that tricks your senses into perceiving something that isn't really there, or isn't the way it appears. When a magician makes a coin seem to vanish into thin air, that's an illusion: the coin didn't actually disappear, but the magician's skilled movements fooled your eyes. When you see water shimmering on a hot road ahead but find nothing there when you reach that spot, you've experienced a mirage, a type of optical illusion caused by how light bends through hot air.

Illusions work because our brains make quick assumptions about what we're seeing, and clever tricks can exploit those assumptions. In a fun house with mirrors, you might look impossibly tall or bizarrely wide. Those reflections are illusions created by curved glass. Artists sometimes create illusions too: a flat painting might look like it has depth, or a sidewalk chalk drawing might appear to be a deep hole in the ground.

The word can also describe false beliefs or mistaken impressions. If you're under the illusion that you can skip studying and still ace a test, you're fooling yourself. When someone says “don't have any illusions about how hard this will be,” they mean you should see the situation clearly, not imagine it will be easier than it really is.

Illusionist is another word for a magician who performs tricks, someone who deliberately creates illusions to entertain audiences.