The credit card security code is a unique three or four digit number printed on your credit card. This number is not embossed on the card and, therefore, not printed on receipts. This makes it difficult for anyone other than the genuine cardholder to know it and it provides an indication that the physical card is in the possession of the buyer/cardholder. Some card issuers refer to this number as the "Card Security Code," "Personal Security Code" or "Card Verification Value." Visa calls it "CVV2," Mastercard calls it "CVC2" and American Express calls it "CID."
Visa, Mastercard and Discover cards have a three-digit number printed at the top, right side of the signature strip, on the reverse of the card.
American Express cards have a four-digit number printed above the account number, on the front, right side of the card.