Eighteen is the numerical value of the Hebrew word "chai" which means "life." It is a Jewish custom to give monetary gifts in increments of 18, thus symbolically blessing the recipient of the gift with a good long life.

When giving charity, the number 18 has another significance. It expresses our prayer that the merit of the charity given stand in our good stead, that we be blessed with life and prosperity.

Chani Benjaminson
Chabad.org