Chai is a combination of the Hebrew letters Het and Yud which together, mean living.
In Judaism, the word is used as emblem of charity. longevity, and good fortune.
Gematrically, Chai enumerates to 18 and therefore, the number is associated with the life force and the immanent presence of God.
It is entirely due to this symbolism that Jewish charitable giving is traditionally in multiples of 18.