a false impression or belief notion
I have no illusions about my ability
something that deceives or misleads intellectually
a misleading image presented to the vision
perception of an object in such a way that it presents a misleading image
an optical illusion
= hallucination
be under the illusion that
to have the mistaken belief that (something is the case)
illusional adj
[Middle English via French from Latin illusion-, illusio action of mocking, from illudere to mock at, from in-1 + ludere to play]