This is a security code that protects your credit card from possible fraud. The CVV (Card Validation Value) on the VISA, the CVC (Card Validation Code) on the MasterCard, or the CIN (Card Identification Number) on the American Express card, is a 3 or 4 digit code that can usually be found on the back of your credit card.
These code are used exclusively for mail-order transactions (purchases made by phone, fax, letter or over the Internet) that may be requested by the merchant.
How to find your code?
Visa/Mastercard and Discover Users
Flip your card over and look at the signature box. You should see either the entire 16-digit credit card number or just the last four digits followed by a special 3-digit code. This 3-digit code is your Card Security Code.
The American Express Card Identification Number (CIN) is a four digit number found on the front of the U.S. issued American Express Card (including Optima). For the American Express card the four digit number is displayed on the front of the card, just above and to the right of the credit card number. For the Optima card it is displayed on the front of the card, to the left and just above the credit card number. American Express and BillOReilly.com are now using this number for added security and customer protection.