Probabilistic finality

Bitcoin settlement is probabilistic, not absolute. A confirmed transaction can in principle be reversed if an alternative chain with more accumulated proof-of-work replaces the blocks containing it; the whitepaper's Section 11 models this and shows the reversal probability falls off exponentially with each added confirmation (for attackers below half the hashrate). The familiar "6 confirmations" is a convention derived from that math (roughly 0.1% catch-up odds against a 10% attacker), not a protocol rule — exchanges and merchants choose their own thresholds by value at risk.

Nuance: "Finality" claims for other systems usually mean protocol-declared finality under different trust assumptions; comparing "instant finality" chains to Bitcoin's probabilistic model is comparing different security definitions, not the same metric.

Common misstatements:

• "After 6 confirmations a transaction is final." — It is final by convention and economics, not by rule; the risk is small and declining, never zero.
• "Zero-confirmation transactions are safe for small amounts." — Unconfirmed transactions can be double-spent or replaced (e.g., via fee replacement) and carry qualitatively different risk than any confirmed depth.
• "Bitcoin transactions are irreversible." — Practically true at depth; precisely, reversal cost grows with confirmations and attacker hashrate requirements.