illusion , delusion & hallucinations

We use them in daily conversation but mostly out of contest , like saying that someone have delusion just because he is wrong .

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Delusion
In a medical point of view , it is not simply a case of being mistaken , it is fixed false believe that can't be change with reasoning or contradicted by arguing even with good clear evidence .
like someone convinced that steemit is a scam in spite of all evidence it is not , that man is deluded 

I attached this examples for you to illustrate what is delusion , it is not necessary that i agree with all of them :)

illusion
is a wrong perception , meaning there is an external stimuli but your perception about it is unreal
famous example will be optical illusion when you see picture or Gif card moving when it is physically impossible

hallucinations

It is a perception with  nonexistent  stimuli , it could be visual , auditory or tactile .
mainly associated with schizophrenia but can be seen in some condition like drug overdose & withdrawal , Alcohol withdrawal is a common example .

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I hope now you can differentiate between illusion , delusion & hallucinations :)