How to spell illusion

1a(1) : a misleading image presented to the vision : optical illusion. (2) : something that deceives or misleads intellectually. b(1) : perception of something objectively existing in such a way as to cause misinterpretation of its actual nature. (2) : hallucination sense 1.

There are three main types of optical illusions including literal illusions, physiological illusions and cognitive illusions. All three types of illusions have one common thread.

Illusion, a misrepresentation of a “real” sensory stimulus—that is, an interpretation that contradicts objective “reality” as defined by general agreement. An illusion is distinguished from a hallucination, an experience that seems to originate without an external source of stimulation.