Arithmancy is the art of Divination by numbers (sometimes wrongly called Arithmomancy).
The ancient Greeks examined the number and the values of letters in each name of two combatants. They predicted the combatant having the name of the greater value would be victorious. It was by using this science that some diviners foretold that Achilles would defeat Hector.
The Chaldeans also practiced arithmancy. They divided their alphabet into three parts, each part composed of seven letters which they attributed to the seven planets. Through this arithmetic method they made predictions based on the planets.
The Platonists and Pythagoreans were also strongly attracted to this form of divination which is similar to certain aspects of the Jewish Kabbalah.
Arithmancy is a magic topic mentioned in many Harry Potter Books and movies.