The term "illusion" refers to something that is intended to deceive the senses or mislead perception.
Illusions: A Way to Trick the Eye
Some examples of illusions are:
M.C. Escher's artwork that is created to visually challenge the audience
A magician who cuts in half a box that has a person inside without cutting the person
Artwork that appears to be only various shapes and colors that changes when one stares at the picture for a period of time
A picture of an old woman looking downward which, when vision is adjusted, appears to be a young woman looking sideways
The popular picture of a goblet that, when vision is readjusted, appears to be two faces looking at each other
A picture consisting of many different colored and sized ovals making up larger circles. When viewed, though motionless, the circles appear to rotate.
A person who is walking on the ground can appear to be walking up a wall when the picture is rotated.
Watching a ventriloquist is an illusion. The audience perceives the puppet as actually speaking, as opposed to the puppeteer.
Mimes create the illusion of doing things within spaces that do not exist, such as walls, or climbing of stairs or ladders.
The Shepard tone which appears to continually ascend and descend in pitch but actually does not.
The Tritone Paradox illusion discovered by Diana Deutsch. Some hear notes that are ascending while others hear notes that are descending.
The Scale Illusion, in which listeners hear different scales in each ear as a result of the brain grouping notes together, was discovered by Diana Deutsch.
Phantom Words is another illusion in which meaningless sound is overlapped over itself several times. While there are no real words, listeners may begin to hear words or phrases that apply to what is currently relevant in their life. This auditory illusion was first demonstrated by Diana Deutsch at the University of California.
The MgGurk effect is an auditory/visual illusion that confuses the brain into thinking a man is repeatedly saying "BA." Most individuals hear "DA" while his mouth appears to be saying "GA."
The phantom limb, the feeling that an amputee might have that his amputated limb is still attached.
The cutaneous rabbit illusion which involves tapping different parts of one's arm to create the feeling of sequential taps up and down the arm.
The Aristotle illusion which occurs when one crosses her fingers and touches one object, such as a carrot or a pen. The illusion is that the individual actually feels she is touching two of the same object instead of just one.
Write on the chalkboard without earplugs, then erase and write again using earplugs. The latter situation will result in it feeling like the chalkboard is smoother. This is because the writer cannot hear chalk squeaking on the board.
Looking at these different examples of illusions can help you better understand illusions you may encounter.