A thaum is generally accepted to be the smallest particle of magic, and therefore the force of any given spell is measured in thaums. In an area where a lot of powerful magic took place, thaums may remain, much like radioactivity.
It has recently been discovered that thaums are made up of resons (Lit.: thingies). Resons themselves are made up from a combination of at least five "flavours", known as 'up','down',sideways','sex appeal' and 'peppermint'.