Chai

Chai (Hebrew: חי‎) is a Hebrew word that means "alive" or "living." It is pronunced as [χai̯].

It is used a symbol, commonly seen on necklaces, and on other jewelry and ornaments. It reflects Judaism's focus on the importance of life. Indeed, the typical Jewish toast is "l'chayim" (meaning "to life").

The word is made up of two letters: Cheit (ח) and Yod (י). Since every Hebrew letter has a numerical value, the letters of chai add up to 18. Therefore, this number has a special meaning in Judaism. Money gifts and donations are usually given in multiples of 18.

You can read more about it here.