Security code is an important new Internet security feature that now appears on the back of most Visa/MasterCard cards. This new code is a three or four-digit number which provides a cryptographic check of the information embossed on the card.
The security code helps validate that the customer placing the online order actually has the credit card in his/her possession, and that the credit/debit card account is legitimate.
Flip your card over and look at the signature box. You should see either the entire 16-digit credit card number or just the last four digits followed by a special 3-digit code. This 3-digit code is your Card Security Code.