Gematria or gimatria (Hebrew: גימטריה gēmaṭriyā) is a system of assigning numerical value to a word or phrase, in the belief that words or phrases with identical numerical values bear some relation to each other, or bear some relation to the number itself as it may apply to a person's age, the calendar year, or the like. It is likely that the term derives from the order of the Greek alphabet, gamma being the third letter of the Greek alphabet (gamma + tria). Alternatively, the word "gematria" is generally held to derive from Greek geōmetriā, "geometry", which was used as a translation of gēmaṭriyā, though some scholars believe it to derive from Greek grammateia, rather; it's possible that both words had an influence on the formation of the Hebrew word. It has been extant in English since the 17th century from translations of works by Giovanni Pico della Mirandola. Although ostensibly derived from Greek, it is largely used in Jewish texts, notably in those associated with the Kabbalah.
The best-known example of Gematria is the Hebrew word Chai ("life"), which is composed of two letters which (using the assignments in the Mispar gadol table shown below) add up to 18. This has made 18 a "lucky number" among Jews, and gifts in multiples of 18 are very common among Jews.
Some identify two forms of gematria: the "revealed" form, which is prevalent in many hermeneutic methods found throughout Rabbinic literature, and the "mystical" form, a largely Kabbalistic practice.
Though gematria is most often used to calculate the values of individual words, psukim (Biblical verses), Talmudical aphorisms, sentences from the standard Jewish prayers, personal, angelic and Godly names, and other religiously significant material, Kabbalists use them often for arbitrary phrases and, occasionally, for various languages. A few instances of gematria in Arabic, Spanish and Greek, spelled with the Hebrew letters, are mentioned in the works of Rabbi Abraham Abulafia; some Hasidic Rabbis also used it, though rarely, for Yiddish. However, the primary language for gematria calculations has always been and remains Hebrew and, to a lesser degree, Aramaic.
Read more about Gematria: Textual Sources, Values Table, Vowels, Methods, Absolute Value, Use in Other Languages