The CVV Number ("Card Verification Value") on your credit card or debit card 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. You can find this number on the back of your credit card/debit card.
Providing your CVV number to an online merchant proves that you actually have the physical credit or debit card - and helps to keep you safe while reducing fraud.
CVV numbers are NOT your card's secret PIN (Personal Identification Number).
You should never enter your PIN number when asked to provide your CVV. (PIN numbers allow you to use your credit or debit card at an ATM or when making an in-person purchase with your debit card or a cash advance with any credit card.)
CVV numbers are also known as CSC numbers ("Card Security Code"), as well as CVV2 numbers, which are the same as CVV numbers, except that they have been generated by a 2nd generation process that makes them harder to "guess".