The Card Security Code (CSC) is the 3 or 4 digit number on credit cards that is intended to reduce fraud by adding an additional layer of security.
Visa: The 3-digit number located on the back of the card.
MasterCard: The 3-digit number located on the back of the card.
Discover: The 3-digit number located on the back of the card.
American Express: The 4-digit number located on the front of the card to the right and just above the credit card number.