noun
1. a fantasy or mistaken belief.
He operated under the illusion that the city was a safe place until that day he was attacked.
Your notion that you will make it big in the music industry is just an illusion.
2. a false perception of what one sees, hears, or otherwise senses.
The magician created the illusion in the mind of the audience that the bird had disappeared.
The white walls give an illusion of greater light and space in the small room.
The government took steps to create an illusion of reform, but little real progress was made.