Decline code
The two-character code a card issuer returns when it refuses a payment, and which rarely tells the whole story.
It lives in field 39 of the ISO 8583 message. 00 means approved, 51 insufficient funds, 54 expired card, 14 invalid card number. The most popular refusal, however, is 05, Do not honor, which in practice means the issuer is saying no and declining to elaborate. Issuers keep codes vague on purpose: a precise answer would teach fraudsters which stolen cards are worth another try.
The industry distinguishes hard and soft declines. A hard decline, such as a stolen card, should never be retried. A soft decline might succeed later or with more data, the clearest case being the code demanding strong customer authentication. Card networks fine merchants who keep hammering away at hard declines.