EMV 3-D Secure is the formal name of the protocol more commonly called 3-D Secure 2 (3DS2), the current standard by which a card issuer verifies that the person making an online card payment is the genuine cardholder. The specification is published and maintained by EMVCo, the body owned jointly by the major card networks, and the first version appeared in 2016.
The "EMV" prefix marks the difference from the original 3-D Secure, which Visa developed around 2001 and which each network then ran under its own rules. That first version redirected shoppers to a password page and worked poorly on mobile devices; the networks retired it in 2022. The EMV version was written for apps as well as browsers, sends the issuer far more data about the transaction and device, and allows frictionless authentication, in which low-risk payments are approved with no action by the cardholder.
Card networks implement the specification under their own brand names, such as Visa Secure and Mastercard Identity Check. It is the main means of meeting strong customer authentication requirements for card payments in Europe, and a successful authentication generally brings a liability shift for fraud from the merchant to the issuer.