Delusion: a fixed false belief. Despite evidence that states otherwise, the delusion is upheld.
Hallucination: a sensory experience in the absence of a stimulus. You think you saw something, but your eyes didn't actually see anything. I saw a cat in the bathroom... but there definitely wasn't a cat there.
Illusion: a stimulus is present but the sensory experience is misperceived. You saw a cat in the corner of your eye, but you perceived the sensation (the cat) as something else (a monster!).
|An example of an illusion. Is this an picture of two faces or a vase? Source|
Reality testing: a way to test the level of awareness that a person's hallucination, delusion or illusion is understood to be misperceptions or misinterpretations. Are you aware that you are delusional, hallucinating, or seeing an illusion, or do you think that everything is 'real'?