What is magic?
There is the wizard's explanation... wizards talk about candles, circles, planets, stars, bananas, chants, runes and the importance of having at least four good meals every day.

After a while the Senior Wrangler said, "Do you know, I read the other day that every atom in your body is changed every seven years? New ones keep getting attached and old ones keep on dropping off. It goes on all the time. Marvelous, really."

The Senior Wrangler could do to a conversation what it takes quite thick treacle to do to the pedals of a precision watch.

"Yes? What happens to the old ones?" said Ridcully, interested despite himself.

"Dunno. They just float around in the air, I suppose, until they get attached to someone else."

The Archchancellor looked affronted.

"What, even wizards?"

"Oh, yes. Everyone. It's part of the miracle of existence."

"Is it? Sounds like bad hygiene to me," said the Archchancellor. "I suppose there's no way of stopping it?"

"I shouldn't think so," said the Senior Wrangler, doubtfully. "I don't think you're supposed to stop miracles of existence."

"But that means everythin' is made up of everythin' else," said Ridcully.

"Yes. Isn't it amazing?

There are eight levels of wizardry on the Disc; after sixteen years Rincewind has failed to achieve even level one. In fact it is considered opinion of some of his tutors that he is incapable even of achieving level zero, which most normal people are born at; to put it another way, it has been suggested that when Rincewind dies the average occult ability of the human race will actually go up by a fraction.

That's what's so stupid about the whole magic thing, you know. You spend twenty years learning the spell that makes nude virgins appear in your bedroom, and then you're so poisoned by quicksilver fumes and half-blind from reading old grimoires that you can't remember what happens next.

It was octarine, the colour of magic. It was alive and glowing and vibrant and it was the undisputed pigment of the imagination, because wherever it appeared it was a sign that mere matter was a servant of the powers of the magical mind. It was enchantment itself.
But Rincewind always thought it looked a sort of greenish-purple.

We've strayed into a zone with a high magical index,' he said. 'Don't ask me how. Once upon a time a really powerful magic field must have been generated here, and we're feeling the after-effects.'

Precisely,' said a passing bush.