Numerology (known before the 20th century as arithmancy) is the belief in an occult, divine or mystical relationship between a number and one or more coinciding events. It is also the study of the numerical value, via an alphanumeric system, of the letters in words and names. When numerology is applied to a person's name, it is a form of onomancy. It is often associated with astrology and other divinatory arts.

Number symbolism is an ancient and pervasive aspect of human thought, deeply intertwined with religion, philosophy, mysticism, and mathematics. Different cultures and traditions have assigned specific meanings to numbers, often linking them to divine principles, cosmic forces, or natural patterns.

The term arithmancy is derived from two Greek words – arithmos (meaning number) and manteia (meaning divination). "Αριθμομαντεία" Arithmancy is thus the study of divination through numbers. Although the word "arithmancy" dates back to the 1570s, the word "numerology" was not recorded in English before c. 1907.

The practice of gematria, assigning numerical values to words and names and imputing those values with religious meaning, dates back to antiquity. An Assyrian inscription from the 8th century BC, commissioned by Sargon II declares "the king built the wall of Khorsabad 16,283 cubits long to correspond with the numerical value of his name". Rabbinic literature used gematria to interpret passages in the Hebrew Bible.

The practice of using alphabetic letters to represent numbers developed in the Greek city of Miletus, and is thus known as the Milesian system. Early examples include vase graffiti dating back to the 6th century BCE. Aristotle wrote that the Pythagorean tradition, founded in the 6th century by Pythagoras of Samos, practiced isopsephy, the Greek predecessor of Hebrew gematria. Pythagoras was a contemporary of the philosophers Anaximander, Anaximenes, and the historian Hecataeus, all of whom lived in Miletus, across the sea from Samos. The Milesian system was in common use by the reign of Alexander the Great (336–323 BCE) and was adopted by other cultures during the subsequent Hellenistic period. It was officially adopted in Egypt during the reign of Ptolemy II Philadelphus (284–246 BCE).

By the late 4th century AD, following the Edict of Thessalonica in 380 AD and the subsequent enforcement of Nicene Christianity under Theodosius I, departures from the beliefs of the state church were classified as civil violations within the Roman Empire. Numerology, referred to as isopsephy, remained in use in conservative Greek Orthodox circles, particularly in mystical and theological contexts.

Some alchemical theories were closely related to numerology. For example, Arab alchemist Jabir ibn Hayyan (died c. 806−816) framed his experiments in an elaborate numerology based on the names of substances in the Arabic language.

Numerology is prominent in Sir Thomas Browne's 1658 literary discourse The Garden of Cyrus. Throughout its pages, the author attempts to demonstrate that the number five and the related quincunx pattern can be found throughout the arts, in design, and in nature, particularly botany.