Arithmancy

Arithmancy is a magical discipline that studies the magical properties of numbers, including predicting the future with numbers and numerology. Those who practice Arithmancy are called Arithmancers.

Arithmancy is an elective subject offered from third year on at Hogwarts School of Witchcraft and Wizardry, the homework assignments of which include writing essays that require the consulation and/or composition of complex number charts.

Notable Arithmancers

 * Bridget Wenlock - the archetypal example; she is a celebrated witch who first discovered the magical properites of the number seven in the 13th century.
 * Septima Vector - teachers Arithmancy and Advanced Arithmancy Studies at Hogwarts School of Witchcraft and Wizardry for at least six years.
 * Hermione Granger - famous high-ranking official in the Department of Magical Law Enforcement, who strongly contributed to Voldemort's downfall as she accompanied Harry Potter in the hunt for horcruxes; translated The Tales of Beedle the Bard into English from Runic.
 * Percy Weasley
 * Bill Weasley
 * Barty Crouch, Jr. - Death Eater who made Lord Voldemort's second rising possible.

Arithmantic texts

 * Numerology and Grammatica, a third year textbook at Hogwarts
 * New Theory of Numerology, which Harry Potter gave to Hermione Granger for Christmas in 1995

Etymology
Arithmancy and Arithmancer come from the Greek αριθμός (arithmos), meaning "number", and μαντεία (manteia), meaning "divination". Hence, Arithmancy literally means "number divination", which is fitting.