Many of the banks that issue cards for Visa will let you get a temporary card number for online shopping. This is how it works: You want to buy something online (for instance, at 12345.com), you just go to your Visa account and request a temporary number. Then you use that number, not your permanent one, to complete the order at 12345.com. It can only be used once, and Visa issuer is the only entity that knows it is temporary and that it is tied to your real number. If someone hacks into 12345.com and steals names/credit card numbers of their customers, the one that you gave to complete your order is useless.
Worried About Online Security? Use a Temporary Credit Card Number | Credit.com News + Advice