Validation Code is the card verification value code.  It is called CVV2 by Visa, CVC2 by Master Card, and CID by American Express.

To aid in the prevention of fraudulent credit card use, we recommend you to provide the Validation Code for card processing.  You can locate the Validation Code as follow.  For Visa/MC it's the 3 digits in the back of the card.  For American Express it's the 4 digits in the front of the card.