ECI (Electronic Commerce Indicator)
A two-digit value in the authorization that tells the issuer how thoroughly an online purchase was authenticated.
After a 3-D Secure check the merchant gets back an ECI, which has to travel with the authorization message. For Visa, Amex and JCB, 05 means the cardholder was fully authenticated, 06 means authentication was attempted but the issuer didn't take part, and 07 means no 3DS at all. Mastercard uses 02, 01 and 00 for roughly the same thing, because card networks, as a matter of principle, never share a numbering scheme.
The number looks trivial but decides who pays if the purchase turns out to be fraud. A 05 or 02 shifts liability to the issuer; a 07 leaves it with the merchant. The value is backed by a cryptographic receipt, the CAVV, so a merchant can't just type 05 and hope for the best.