A CV2 or CVV2 number is a three-digit number printed on the back of most credit cards, adjacent to the signature space. Visa, MasterCard and Discover cards have it printed as a form of added security measure.
The CV2 number is not part of the card number that is embossed on the front. CVV stands for Card Verification Value and is frequently requested for completing online transactions. It is used to validate that the card is in possession of the owner and that it is authentic. American Express cards have the CV2 number printed on the front, just above the credit card number.