CAVV
The cryptographic proof from 3-D Secure that the issuer checks to know the authentication really happened.
The Cardholder Authentication Verification Value is generated by the issuer's Access Control Server once the customer has passed the 3DS step. The merchant includes it in the authorization, and the issuer verifies it with a key only it holds. The value binds together the card, the transaction and the authentication result, so it can't be lifted and replayed on another purchase.
The name is Visa's. Mastercard calls the same thing AAV and carries it in a field called UCAF; Amex calls it AEVV. Without the CAVV, the ECI would be nothing more than the merchant's say-so. With it, the liability shift becomes something that can be tested after the fact, which is all the networks really care about.