Hallucinations, delirium, and hysteria are all different things. Hysteria is a bit of a controversial, antiquated medical condition which used to be attributed to anyone who was a bit "out of sorts" especially if they were female. Nowadays hysteria is generally used to refer to a state of panic or extreme excess of stress. Delirium is a condition where the persons perceptions have been disturbed removing the ability to think coherently and often causing hallucinations. This can be caused by Drugs, Alcohol, lack of sleep, or high fevers. Hallucinations are where somebody perceives something which is not there. This could be a sound, image, or feeling. These are generally created through things such as drugs, extreme illnesses, brain damage, high fevers, or lack of sleep. Hope this helps :)

Certainly explained very well, but I would like to add something. If you want to describe someone you would use the word "delirious" or "suffering from delirium." This tends to mean that someone is acting irrational, not thinking clearly, or clearly not coherent. It is not quite as aggravated a state as "hysterical."

I agree with Certainly, they have explained it well :)