Definition
Probabilistic finality describes how Bitcoin settles transactions: there is no single moment at which a payment becomes officially irreversible. Instead, the probability that a transaction could ever be reversed falls exponentially as more blocks, and therefore more proof-of-work, accumulate on top of it. A transaction is never absolutely final, only ever "final enough" for a given level of risk.
How it differs from absolute finality
Some consensus systems offer deterministic or quorum-based finality, where a transaction becomes irreversible once a validator threshold signs off. Bitcoin trades that hard guarantee for an open, permissionless network secured by accumulated work and the longest-chain (most-work) rule. The result is rising confidence with depth rather than a binary settled/unsettled flag.
Reading the risk
Because reversal requires re-mining every block above a transaction and out-pacing the honest network, the practical security of a payment is a function of how deep it sits. Six confirmations, roughly an hour, is the long-standing convention for treating a transaction as settled; high-value transfers warrant more. The deeper the burial, the more astronomically expensive any rewrite becomes.
Probabilistic finality is the reason merchants count confirmations rather than waiting for a single settlement event, and it is the property that a deep reorg from a 51% attack would have to defeat to undo a payment.
In Simple Terms
Probabilistic finality describes how Bitcoin settles transactions: there is no single moment at which a payment becomes officially irreversible. Instead, the probability that a transaction…