Authorization reversal
The merchant telling the issuer that an approved hold won't be used, so the funds become available again immediately.
Without a reversal, an unused hold sits there until it expires, often a week later. That is how a cancelled hotel night or an abandoned order keeps locking up a customer's money. A reversal, its own message type in ISO 8583, releases the amount at once. It can be full, when the order is cancelled, or partial, when the fuel pump reserved more than the tank could hold.
It is not a refund. A refund moves money that has already been captured; a reversal makes sure it never is. Visa and Mastercard charge merchants who leave authorizations dangling, since stale holds tie up customers' balances and muddy the issuers' risk picture.