A hallucination is seeing, hearing, tasting, smelling or feeling things that do not really exist. Usually, people have to be awake and conscious to have hallucinations. That way, a person who sees or hears something that is not really there has a hallucination. Hallucinations are different from dreams. When people dream, they are not awake. Hallucinations are also different from illusions. Illusions are based on real perceptions, which are distorted or interpreted in a wrong way.
Certain drugs can cause hallucinations. Some illegal drugs are taken because they cause hallucinations. Certain mental illnesses can also cause hallucinations. In some cases, withdrawal of a drug can also cause hallucinations. This has especially been reported by people trying to stop taking sleeping pills, or by alcoholics.
People who have hallucinations are not necessarily ill, lack of sleep can also cause hallucinations. Hallucinations just before going to sleep or just after waking up are considered normal.