The Card Security Code (CSC), sometimes called Card Verification Value, is a security feature for credit card transactions, giving protection against credit card fraud. This number should not be confused with the standard credit card number appearing in embossed digits.
Where is it?
MasterCard has a 3 digit code, called the "CVC2" (card validation code) and Visa cards have the "CVV2" (card verification value). It is not embossed like the card number, and is always the final group of numbers printed on the back signature panel of the card.
New MasterCard and Visa cards feature the "CVC2" in a separate panel to the right of the signature strip. This has been done in order to prevent overwriting of the numbers by signing the card.
What is this code important?
This code is separate from the credit card number and is intended to verify that the customer has the card in their physical possession.