Card verification value (CVV)
The short check code that ties a card number to a physical property of the card.
CVV comes in two variants. CVV1 is encoded on the magnetic stripe and is derived from the card number, expiry date and service code under a key only the issuer holds. CVV2 is printed on the back of the card and is used for payments where the card is not present. Mastercard calls the equivalent CVC2.
Because CVV2 is printed on the card itself, it does nothing against someone who has the card in hand or a photo of it. That is why it is supplemented by measures such as 3-D Secure, and none of them is perfect.