Meaning of “illusion” - Learner’s Dictionary

illusion

noun uk us /ɪˈluːʒən/
FALSE IDEA [ C, U ]

an idea or belief that is not true:

He had no illusions about his talents as a singer.
We are not under any illusion - we know the work is dangerous.
DIFFERENT [ C ]

something that is not really what it seems to be:

There is a large mirror at one end to create the illusion of more space.
→ See also optical illusion

(Definition of “illusion” from the Cambridge Learner’s Dictionary © Cambridge University Press)