CVV2 is an important security feature for credit card transactions on the Internet and over the phone. "CVV" stands for "Card Verification Value" (Discover Card calls it the "Cardmember ID").

CVV2 & CVV - Discover, Mastercard, Visa Discover, Mastercard, Visa

It is the three-digit number printed in the signature space on the back of most credit cards, such as Visa, Mastercard, and Discover cards. The CVV2 number is always the last group of numbers in the signature space on the back of the card. It is not part of your regular credit card number.
CVV2 & CVV - American Express American Express

It is a four-digit number on the front of American Express cards. It is printed (flat), not embossed like the card number.