Numerology (known prior to 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 the paranormal, alongside astrology and similar to divinatory arts.

The term numerologist can be used for those who place faith in numerical patterns and draw inferences from them, even if those people do not practice traditional numerology. For example, in his 1997 book Numerology: Or What Pythagoras Wrought (Dudley 1997), mathematician Underwood Dudley uses the term to discuss practitioners of the Elliott wave principle of stock market analysis.