Gematria (/???me?tri?/; Hebrew: ?????? or gimatria ???????, plural ??????? or ?????????, gimatriot) is the practice of assigning a numerical value to a name, word or phrase according to an alphanumerical cipher. A single word can yield several values depending on the cipher which is used.

Hebrew alphanumeric ciphers were probably used in biblical times, and were later adopted by other cultures. Gematria is still widely used in Jewish culture. Similar systems have been used in other languages and cultures: the Greeks isopsephy, and later, derived from or inspired by Hebrew gematria, Arabic abjad numerals, and English gematria.

Although a type of gematria system ('Aru') was employed by the ancient Babylonian culture, their writing script was logographic, and the numerical assignations they made were to whole words. The value of these words were assigned in an entirely arbitrary manner and correspondences were made through tables, and so cannot be considered a true form of gematria. Aru was very different from the gematria systems used by Hebrew and Greek cultures, which used alphabetic writing scripts. Similar systems have been used in other languages and cultures derived from or inspired by Hebrew gematria; Arabic abjad numerals, and English gematria. There is currently no academic consensus over whether Hebrew gematria or Greek isopsephy was used first.

Gematria sums can involve single words, or a string of lengthy calculations. A well-known short example of Hebrew numerology that uses a gematria cipher is the word ?? chai (lit. "alive"), which is composed of two letters that (using the assignments in the Mispar gadol table shown below) add up to 18. This has made 18 a "lucky number" among the Jewish people. Donations of money in multiples of 18 are very popular.

In early Jewish sources, the term "gematria" can also refer to other forms of calculation or letter manipulation, for example atbash. [Wikipedia]