Well, Jos, and Red, I can see the point (It's blue, and in the key of A minor;-))

That's certainly my naive understanding of my own synesthetic experience, which was under the influence of a hallucinogen. (I saw the succession of musical stimuli as cascading jewel-like streams that completely filled my visual field.)

The Random House dictionary defines "hallucination" as

"a sensory experience of something that does not exist outside the mind, caused by various physical and mental disorders, or by reaction to certain toxic substances, and usually manifested as visual or auditory images."

But in a way, that's question-begging, because a) colors and sounds per se only exist inside the mind anyway, and b) those who've experienced synesthesia throughout their lives would certainly dispute its categorization as the result of "disorders."

On the other hand, I had a visual experience with nitrous oxide that consisted of Mandelbrot fractals continually generating themselves more and more complexly--again, extraordinarily vivid and completely filling my visual field.

To me, there's very little difference between the two experiences. One just came from an internal stimulus, perhaps a memory, perhaps an intuitive understanding of the generating principles. Both involved visual interpretation of non-visual mental patterns--due to the influence of hallucinogens. So for me, there's not much distinction, but I understand how for others there may well be.