The Card Security Code provides an additional security measure to prevent card misuse within on-line transactions. (The Card Security Code is also known as the Card Verification Value.)
For the majority of credit cards, the security code is the last three digits of the number shown above the signature strip on the reverse side of the credit card. In the example below, the Card Security Code is '567'.
For American Express Cards, the security code is the four digit number shown above the credit card number, on the right-hand side of the front of the card.