3-D Secure
The protocol that lets the card issuer check who is actually at the keyboard when a card is used online.
Visa launched it around 2001 as "Verified by Visa", and Mastercard followed with SecureCode. The three "domains" are the issuer's, the acquirer's and the shared plumbing in between. Version one bounced shoppers into a popup and asked for a static password they had chosen years earlier and long forgotten. The result was abandoned baskets and a gift to phishers, since nobody could tell a genuine popup from a fake one.
EMVCo's 3DS2, published in 2016, turned the idea around. The merchant sends around a hundred data points about the device and the purchase, the issuer scores the risk, and most transactions go through without friction. Only the suspicious ones get a challenge, usually in the banking app. Authenticating through 3DS also shifts fraud liability from the merchant to the issuer, which is why merchants put up with it at all. Version one was switched off in October 2022.