What is the difference between 'illusion' and 'delusion'?

Both of them could mean 'false ideas'. I cannot differentiate them.


  1. a false or wrong belief or idea

Many people still have the illusion that full employment is possible.

I had no illusions about the task that lay ahead.


  1. an idea or belief that is not true

It is my conviction, or my delusion, no matter which, that crime brings its own fatality with it.

He had long known that the delusion was partly due to a trap laid for him by Dunstan, who saw in his brother's degrading marriage the means of gratifying at once his jealous hate and his cupidity.


2 Answers 2


A delusion is a strongly held belief even when there is strong evidence that the belief is false.

An illusion deceives or misleads someone about the true nature of something. Magicians are often called illusionists because they mislead us into believing that something impossible has happened. Most of the audience however is not deluded into believing that the magician actually performed real magic.

For example:

"The positioning of the mirrors gave the illusion that the room was much larger than it actually was."

"If we could dispel the delusion that Engineering requires a passion for mathematics, we might see more women studying it."

  • I've added two examples in my question; it seems to me that "delusion" would be the better word for them. How is that? @ColleenV
    – Kinzle B
    Oct 3, 2014 at 2:28
  • Your first example would be better as delusion, but the second would be better as illusion. "I had no illusions" is expressing that you see clearly what lies ahead, not that you hold a belief that isn't false. "I had no delusions that the meeting would go smoothly." would be better - I am expressing that my belief is that the meeting is going to be difficult, and by using delusion I imply that I have reasons to believe I'm correct. @KinzleB
    – ColleenV
    Oct 3, 2014 at 3:40
  • 1
    Illusion is also used for a false belief, but without the person under the illusion ignoring clear evidence that the belief is wrong.
    – gnasher729
    Oct 8, 2014 at 11:00

Delusion is in the mind of the subject. Illusion is created by an outside actor. Delusion has negative connotations because it implies insanity.

"He had the delusion he was Napoleon"


"The mirror created the illusion that the room was bigger"

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