What is the Card Security Code?
The card security code (CSC) is a 3- or 4-digit number (not part of the credit card number) that is printed on the credit card.
CSC Location for Visa, MasterCard, and Discover
The CSC is the 3-digit number located on the back of the credit card, immediately after the last 4 digits of the credit card number (which is usually in the signature field).
CSC Location for American Express
The CSC is the 4-digit number located on the front of the credit card, on the right or the left above the credit card number.