"If being good at Divination means I have to pretend to see death omens in a lump of tea leaves, I'm not sure I'll be studying it much longer! That lesson was absolute rubbish compared with my Arithmancy class!"
-- Hermione Granger (PA6)
Arithmancy is a branch of magic that is concerned with the magical properties of numbers; someone who practices Arithmancy is called an Arithmancer.
Bridget Wenlock was a famous 13th century Arithmancer. She discovered the magical properties of the number seven (BoS).
Books about Arithmancy include:
"Arithmancy" is derived from the Greek arithmos (number) and manteia (divination). Historically it's the study of numbers connected to the letters of a name or the birthday month in order to foretell someone's future (source: Wikipedia) That makes it quite ironic that Hermione viewed Professor Vector's class as superior to Trelawney's since both are about Divination.