A distorted sensation that is based on the misinterpretation of a real stimulus (for example, a pen is seen as knife). Illusion is different from hallucination, in which an experience occurs without any stimulus. Generally, illusions are short and can be understood when they are explained. They can be caused by anxiety or tiredness, to drugs, or to forms of brain damage. Delirium tremens (a condition often arising from alcohol withdrawal) can bring on illusions.