Your Security Code is a 3- or 4-digit code printed on your credit card which improves the security of using your credit card online:

Visa, Mastercard or Discover Your Security Code is the 3-digit, non-embossed number printed on the back of your card on the signature panel, immediately following your account number.

American Express Your Security Code is the 4-digit, non-embossed number printed on the front of your card, to the right or left above your credit card number.