Difference between 'illusion' and 'delusion'

Question

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

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

illusion

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.

delusion

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.

Answer 1

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."

Answer 2

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"

vs

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