Eighteen is considered a special number in Jewish culture because it is the numerical value of the Hebrew word chai, meaning “life.” In Hebrew, each letter has a numerical equivalent. For example, the first letter of the Hebrew alphabet, aleph, is equal to one. The second letter, bet, has the value of two, and so on until we reach 10, 20…100, 200 and so on. The Hebrew word chai comprises the letter chet, which has a numerical value of eight, and the letter yud, which has a value of 10.
As a result, it is common for Jews to give gifts in multiples of 18, especially for Jewish lifecycle events such as b'nei mitzvah (bar or bat mitzvah) and weddings, or when donating to charitable organizations, especially to Jewish charities.