Thaum

A thaum was a measure of magic defined as the amount of energy required to raise the temperature of a quart of water by one degree. That said, the measurement usually called "thaum" is more appropriately called a kilothaum or a Thaum (notice the capital Eth). A so-temred 1st-Level Spell requires the expenditure of one Thaum.

The symbol for Thaum, as used in mathematical and alchemical notation, was the combination of a  and  graphemes, also known as an Eth. It can be seen in the page image.

Trivia
The concept of thaum is a direct reference to the late Sir Terry Pretchet's thaum, or the basic unit of magic, as it appears in his most excellent Discworld series. The word itself is derived from the Greek θαῦμα, which meant 'miracle.'