alex.tan's writeup is a good description of the scientifically accepted definition, but I find the scientifically accepted definition to be quite silly. First of all, there are many people who demonstrate one or more of the symptoms listed above, who are as sane or many times more sane than the average person. To wit:
  • It is very possible, through many scientifically normal means, to have control over people's minds. See religion, mind cracking, mind hacking, advertising.
  • "believe that there are special personal messages for them in newspapers or on the radio or television."
    Has anyone ever dedicated a song to you?
  • "believe that he/she has a special link to God or indeed IS God."
    This is practically the most often quoted maxim from the more spiritually advanced people on the planet: "Thou art God." In fact, many of them see it as crazy to believe that you are not God.

So what distinguishes a schizophrenic, someone who has a mental illness, from someone who is, to coin a phrase, radically sane?

Here's the way I see it. Let's use the metaphor of a house for the mind -- a common dream metaphor. A mentally normal person stays inside the house all the time -- they see themselves as defined by their ego, the house. The very sane, the spiritually advanced or gifted, are capable of leaving the house and returning. They acknowledge and identify with more than their ego. But in a schizophrenic person, there is something wrong with the house. Sometimes it has completely collapsed, sometimes it's just the equivalent of a window left open in a storm. And the things that lie outside the house -- visions, godforms, archetypes can enter their consciousness without their control or consent.

A famous story about a mystic and a schizophrenic sums it up. The mystic went to visit the schizophrenic in the asylum.

Mystic: "So, they tell me you think you are Jesus."
Schizophrenic: "YES!!! I AM THE WAY!! I AM THE TRUTH!!"
M: "Well, I happen to think that I am Jesus, and most of my friends think they are Jesus too. We like being Jesus together. What do you think of that?"
M: "Well, I guess that's why you're in here, and we're out there."

It's an extreme example of a schizophrenic patient -- not all people diagnosed with schizphrenia behave like this. (Thank you, Trip Technician, for pointing that out -- my bad.) But it shows what I mean, more generally, about mental illness and sanity.

See The Pre/Trans Fallacy for more discussion of this apparent paradox.

BTW, schizophrenia is NOT multiple personality disorder.