1 : something that looks or seems different from what it is : something that is false or not real but that seems to be true or real
The video game is designed to give the illusion that you are in control of an airplane.
They used paint to create the illusion of metal.
She says that all progress is just an illusion.
2 : an incorrect idea : an idea that is based on something that is not true
She had/harbored no illusions about how much work the project would require. [=she knew the project would require a lot of work]
He was under the illusion [=he mistakenly believed] that he was a good player.