Douglas Harper's Etymology Dictionary
1680s, from Hebrew gematriya, from Greek geometria (see geometry). "[E]xplanation of the sense of a word by substituting for it another word, so that the numerical value of the letters constituting either word is identical" [Klein].
Wiktionary
n. A Kabbalistic method of interpreting Hebrew words, based on numerical values of letters.
Wikipedia
Gematria originated as an Assyro-Babylonian-Greek system of alphanumeric code/ cipher later adopted into Jewish culture that assigns numerical value to a word/name/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 Nature, a person's age, the calendar year, or the like.
Similar systems, some of which were derived from or inspired by Hebrew gematria, have been used in other languages and cultures, i.e. Greek isopsephy, Arabic abjad numerals and 'Simple(6,74) English(7,74) Gematria(8,74)'.
The best-known example of Hebrew gematria is the word Chai ("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 Jews, and gifts in multiples of 18 are very popular.
Usage examples of "gematria".
The only thing else he lets you know about the passage is that it contains 30 words and 124 Hebrew letters, and that when the numerical value of these ancient words and letters is added up by a process known as gematria the sum total of the passage equals 5,449, which is the height of the Great Pyramid in pyramid inches.
I return the gift of the Gideons to its drawer and shut it away along with the secret gematria of the Epik Fuck.
We also have gematria, which you probably heard about, assigning numerical value to each letter of the alphabet.