What to KnowA delusion is an idea that is falsely believed (“he was under the delusion that the earth was flat”) while a hallucination is a thing someone believes they can see, hear, or smell that doesn’t, in fact, exist (“the sound of glass shattering in the next room turned out to be a hallucination brought on by lack of sleep”).
Both delusion and hallucination concern tricks played on the mind. However, delusion refers to something intangible, an idea that the mind believes to be true (that in reality is false), while hallucination refers to something experienced by one of more of the senses—touch, smell, hearing, sight, etc. that in reality isn’t there. Here are some examples of how delusion may appear in a sentence:
The growling dachshund seemed to be under the delusion that it was as fearsome as a dog five times its size.
You seem to be under the delusion that I care what you think, and I really do not.
My friend had an advanced engineering degree, but still suffered from the delusion that the moon landing was faked.
Here are some examples of hallucination used in a sentence:
The strangest hallucination I had during my bout with food poisoning was seeing my neighbor mowing his lawn while he quacked like a duck.
His hallucinations included his shampoo smelling like cheese, and his cheese tasting like shampoo.
She could not tell if what she was seeing was real or if it was a hallucination.
Hallucinations are usually described as occurring due to illness or psychoactive substances (such as LSD) but such causes are not strictly required for delusions. Sometimes all that is required for a delusion is an inflated ego. Famously, the original Star Wars trilogy twice made use of a common phrase that includes the word delusion in such a way. In The Empire Strikes Back (1980), the droid C3-PO accuses R2-D2 of having “delusions of grandeur” when the latter sets about trying to fix their starship. In Return of the Jedi, Han Solo says the same about Luke Skywalker when he learns he’s become a Jedi. In both cases, "delusions of grandeur" is used by one character to accuse another of having a false belief about the greatness of their abilities. “Hallucinations of grandeur” wouldn’t make sense in either scene; it is not used in the same way that "delusions of grandeur" is.
