Card Security Code
The Card Security Code is the group of numbers immediately following your credit card number. These numbers are used by your credit card company to help prevent fraud. Because the Card ID number is not printed on your receipts, it helps ensure that someone is not using your credit card information fraudulently.
Visa and Mastercard - last 3 digits on back of card
Note: The digits are printed on your card, rather then stamped in the plastic like the digits in your credit card number.