CVV (or CVC or CSC) number
The CVV Number (“Card Verification Value”) is a 3 digit number on VISA, MasterCard and Discover branded credit and debit cards. On your American Express branded credit or debit card it is a 4 digit numeric code. It can be located on the reverse of the card, at the end of the paper sticker provided for signature of the card holder. They are generated by a 2nd generation process that makes them hard to “guess”
CVV numbers are not to be confused with secret PIN of the card. It is not to be entered in the ATM or PoS devices and is used as a security feature, in situations where a PIN cannot be used.While making e-commerce payments, the Bank takes control of payment process and presents its own screen in the browser, prompting the buyer to enter the CVV. The PIN should not be entered here.
CVV besides being a security feature, it also serves as a means of the Issuing Bank finding out, if the transaction was indeed a Card Present (CP) transaction and not a Card Not Present (CNP) one; that is, whether the card holder was aware of the transaction or not.
CVV numbers have different nomenclatures: (https://en.wikipedia.org/wiki/Card_security_code)
CID: “Card Id”, “Card Identification Number”, or “Card Identification Code” – Discover, American Express (four digits on front of card)
CSC or “Card Security Code” – debit cards, American Express (three digits on back of card)
CVC2 or “Card Validation Code” – MasterCard
CVD or “Card Verification Data” – Discover, sometimes used as the common acronym for this kind of code
CVE or “Elo Verification Code” – Elo in Brazil
CVN2 or “Card Validation Number 2” – China Union Pay
CVV2 or “Card Verification Value 2” – Visa
The CVV for each card is generated by the card issuer when the card is issued. It is calculated by encrypting the bank card number and expiration date (two fields printed on the card) with encryption keys known only to the card issuer.