Issuer
The bank that gave the shopper their card, as opposed to the acquirer that serves the merchant. The two halves of a card transaction are called issuing and acquiring, and most institutions do one of them. The issuer decides whether to approve a payment; the acquirer cannot overrule that decision.
Why it matters to a merchant
A merchant never contracts with an issuer and will still spend time on them, because the issuer is who actually says no. Most declines at a counter are the issuer's decision, made on the cardholder's account rather than on anything the merchant did, and the terminal is only reporting it. Knowing that changes how staff handle it: the fix is the cardholder calling their bank or trying another card, not the merchant retrying the same card until something changes.
Where this comes up
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Related terms
In this section: Who does what