You have an illusion when your perception is distorted. You believe something that does not exist fro instance. Optical illusions are often used par illusionists -or a conjurers- as to mislead the audience and fill it with wonder when they reveal the truth. We have all seen, on TV or during a sight, a man cutting a woman in 2 halves, and the same woman to eventually come back intact after a veil has passed over her! Or these attached, enchained and cooped up people who manage to get out of this in one minute! All these situations are playing on illusion and the fact that the audience does not perceive reality like it is really but is misled by what it sees.

Some optical illusions are also very famous, like the Müller-Lyer's one, that consists in drawing 3 parallel segments of the same length, in adding to their extremities some arrows pointed towards the inside for the 1st one and the third one, and towards the outside for the 2nd. Then we believe that the 2nd one is longer than the others, but it's just an illusion!

Mirage also are a kind of optical illusion. We think that we see water a few inches further, and this illusion disappear as we come closer.

Having an illusion is also to believe in one thing that is wrong for sure. So we are said to be hedged, and we can believe an assumption is the truth, even after counter-arguments have been found, just because we prefer to occult the latter and keep believing in the assumption. For instance, believing someone is guilty whereas he has been proved innocent is to delude oneself.