Chai (Hebrew: חַי "living" ḥay) is a Hebrew word that figures prominently in Jewish culture; the Hebrew letters of the word are often used as a visual symbol.
The word is made up of two letters of the Hebrew alphabet - Chet (ח) and Yod (י), forming the word "chai," meaning "alive," or "living." The most common spelling is "Chai", but the word is occasionally also spelled "Hai". The usual modern pronunciation of this word is [χai̯], while a transcription of the Biblical and Mishnaic pronunciation would have likely been [ħai̯] (with a pharyngeal consonant).
There have been various mystical numerological speculations about the fact that, according to the system of gematria, the letters of chai add up to 18 (see "Lamedvavniks" etc.). For this reason, 18 is a spiritual number in Judaism, and many Jews give gifts of money in multiples of 18 as a result.