It is the three-digit number printed in the signature space on the back of most credit cards (Visa, Mastercard, Discover, Diners Club, Carte Blanche cards, etc.).The Security Code 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.
It is a four-digit number on the front of American Express cards. It is printed (flat), not embossed like the card number.