Quantifying Magical Energy

The Thaum is a measuring unit used in the Terry Pratchett series of Discworld novels to quantify magic.  It equals the amount of mystical energy required to conjure up one small white pigeon, or three normal-sized billiard balls.  It can, of course, be measured with a thaumometer, and regular SI Prefixes apply (e.g. millithaum, kilothaum).

A thaumometer looks like a greenish glass cube with a dial on one side.  A standard one is good for up to a million thaums — if there is more magic than that around, measuring it should not be your primary concern.

Parodying the introduction of the metric system later Discworld novels refer to the introduction of the newer unit Prime to avoid arguments around standard sizing of pigeons.  It is more reliably defined as the magical energy required to move one pound of lead one foot.

It is not to be confused with the magical particle "thaum" from the same series of novels.