What is a Credit Card ID or Card Verification Value?
The CID (card identification) number is the three or four digit code which helps e-commerce merchants validate that the customer has a credit card in his or her possession, and that the card account is legitimate.
Visa, Mastercard, and Discover

American Express

The Card ID is usually on the back of the card, following all or the end of your account number.

The Card ID is usually on the front of the card, above and right of your account number.