What Hallucinations are

"Most people have a rough idea of what a hallucination experience might be like, but when it comes to defining a hallucination, that's more difficult. If a hallucination is defined as 'seeing or hearing something that is not actually there,' then dreams and imagery would be considered hallucinations.

According to ffytche, visual hallucinations, (people do hallucinate with other senses), "are located in the world around us, not in the mind's eye. They are not under our control, in the sense that we cannot bring them on or change them as they occur. They also look real and vivid, although the things one sees may be bizarre and impossible."

I think I have a simpler explanation of hallucinations.
I do not agree that they are "located in the world around us, not in the mind's eye".
It is exactly the opposite.
Assuming that we do not see the world, but we see the world reflected by our eyes (infact just consider how much shaper is a vision with glasses and how different reality can look seen under the lenses of a microscope) hallucinations are nothing else than a distorted transmission of what our eyes see or our ears hear or what our nose smells.
If you send to the brain stimulus in a different way or if you distort the way stimulus are sent, you have hallucinations.
You look at the same thing, but the conditions and the transmission of what you see is different.

The same can happen without hallucinations, just with the brain conditioned by what we read or know.
How much different a music sounds to our ears when we know it, or a painting looks when we actually are explained about it.
I used to go through museums looking at masterpieces and seeing nothing of them.
The thing changed when I actually had a guide who explained me what I had to see.
"the great poet is always a seer, seeing less with the eyes of the body than he does with the eyes".

One could come to the conclusion that the world is a collective hallucination.
We see what we see because we have a certain kind of eyes.
Or we see what we are meant to see.
Because we still have to answer to the same question: what are we, who made us this way, what are we here for?