The Card Security Code is an added security feature to help protect you against online fraud.
The Card Security Code is the group of numbers immediately following
your credit card number on the back of the card. These numbers are
used by your credit card company to help prevent fraud. Because the Card
Security Code is not printed on your receipts, it helps ensure that someone
is not using your credit card information fraudulently.
Visa, Mastercard, and Discover
For Visa and Mastercard there are 3 digits on the back of the card in the area shown by the graphic above.
For American Express there are 4 digits on the front of the card in the area shown by the graphic above.