A Thaum is the basic unit of magical strength. It has been universally established as the amount of magic needed to create one small white pigeon or three normal-sized billiard balls.

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.

Perhaps the magic would last, perhaps it wouldn't. But then again, what does?

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.