The security code on a credit card (indicated as CVV) is a validation code that verifies the authenticity of your Visa, MasterCard, Diners Club or American Express card. On Visa, MasterCard and Diners Club cards this security code is on the reverse side of the card and is identified by the last 3 numbers. On American Express cards it is a 4-digit code printed on the front of the card, just above the card number.