An illusion is something that isn't real. It may look real, but it's actually fake — just a crafty construction or fantasy. Like the old rabbit-out-of-the-hat trick practiced by magicians around the globe.
An illusion is an act of deception. Some optical illusions are pretty cool to watch, but an illusion can also point to an erroneous belief or false perception of reality, which is where you start getting into hallucination territory — seeing things that aren't there. You can give the illusion that you’re fascinated by your professor’s lecture by chewing on your pencil, furrowing your brow, and making sure to nod enthusiastically every so often.