An illusion is by definition something which doesn't exist. An illusion as a description refers to a false image, concept, or view of a subject. In common usage, the word is also used to describe a false state of reasoning based on an illusion. Metaphorically, an illusion is generally based on the logic of a situation.
Examples of Illusion:
A mirage is an illusion.|
Belief in the ethics of politicians is sometimes referred to as mass illusion, although mass delusion would also work.
Illusionists use visual tricks to persuade people that something that didn't happen is real.
I would suggest that you're living an illusion, and can't really lay eggs and build a nest with your nose.
An optical illusion. Square A is exactly the same shade of grey as Square B.