“Growing up, my parents used to give money gifts in multiples of 18. I know this number, chai, has a special significance to the Jews, but I don’t know why. What’s so special about it?”
Jews love life. I, of course, do not mean to imply that those who are not Jewish do not love life, but I do mean to say that a love for life is a fundamental focus of the Jewish tradition. The Torah begins with recounting the genesis of life in our universe. The first of the 613 commandments is the mitzvah to bring new life into this world. A “long life” is the only reward ever mentioned in the Torah for the performance of mitzvot. Other than in very extreme circumstances, the saving of a life supersedes every mitzvah in the Torah.
This focus on life also finds expression in Jewish culture and customs. When Jews drink alcohol, we say to each other, “L’chaim!”—which means “To life!” On a person’s birthday we traditionally do not say “Happy birthday,” but rather, “May you live till 120!” And, of course, the custom that you referenced in your question, namely to give to charity in multiples of 18. Why 18? All the letters of the Hebrew alphabet have a numerical value attached to them. Aleph is 1, Bet is 2, Yud is 10, etc. The Hebrew word for “life” is “chai.” That’s chet (8) and yud (10). Thus 18 stands for the “chai” or “life.”
This custom of giving charity in multiples of 18, relative to other Jewish customs, is quite recent. It does seem to have started with the Kaparot ritual performed on the day before Yom Kippur. Simply speaking, charity is given as close as possible to Yom Kippur in the hope that in the merit of giving tzedakah, the individual will be “sealed in the Book of Life.” Thus began the custom to give 18 (chai) dollars (or zlotys, rubles, etc.) for the Kaparot charity.
The chai amount clearly caught on and, at some point at the beginning of the 20th century, Jews started using chai for all forms of charity. I believe that the extension of the custom reflects the verse in Leviticus that tells us that giving financial support to someone in need brings that person to life, as it were.
Interestingly, the Hebrew word for death is “mavet,” which has the numerical value of 446. The Satmar Rebbe (1887-1979) is quoted as saying, “Wouldn’t it be great if people gave tzedakah corresponding to mavet and not chai?”