Keep Learning

This three-digit number is printed on the back of the customer's card, directly after the card number on the signature strip.  Online merchants usually require customers to supply this code when they buy something from them.  It is an additional security measure that proves the customer is the rightful owner of the card.  Sometimes, even if a customer orders takeout, the restaurant might ask for the code for authorization purposes. 

Merchants are prohibited from saving the codes; they are just used to validate immediate transactions.