Zero-confirmation transaction
A zero-confirmation transaction (also called a 0-conf payment) is a Bitcoin payment that a receiver treats as settled before any block includes it. The receiver sees the transaction in the mempool or on the P2P network and may release goods or credit an account immediately. Settlement under that policy is provisional. A conflicting spend of the same inputs can still be mined instead, leaving the receiver unpaid.
Bitcoin's consensus finality is tied to block inclusion and subsequent work. Early merchant practice sometimes accepted unconfirmed payments for small face-to-face purchases under a first-seen heuristic. Replace by fee policies weakened those assumptions. BIP 125 defined opt-in signaling with nSequence and replacement fee rules focused on higher absolute fees and incremental payment for relay bandwidth. It did not prohibit replacing non-signaling transactions at consensus. Bitcoin Core later added full RBF settings that allow replacement regardless of signaling. As full RBF became widespread default policy, any unconfirmed payment could be displaced by a higher-paying conflict that honest miners prefer. The Lightning Network and other off-chain protocols exist in part because on-chain 0-conf cannot provide fast, high-assurance settlement for open-network commerce. After one confirmation, reversing the payment requires a chain reorganization.