Chai: "Living"
The numeric value of the Hebrew word Chai is 18 and this may be the reason why gifts to charity are routinely given in multiples of 18.
This symbol which we often come across on necklaces, bangles, and other jewelry and ornaments, is simply the Hebrew word Chai (living). It combines two Hebrew letters Cheit and Yod attached to each other. There is also the view that it refers to the Living G-d; but another prevalent opinion is that it simply reflects the focus of Judaism on the importance of life itsef. In any case one has to admit that the concept of chai is important in Jewish culture. Remember the typical Jewish toast: l'chayim (to life).
Gifts to charity are routinely given in multiples of 18 the numeric value of Chai
-Prof. George Menachery