Finality

Finality describes when a transaction becomes irreversible. On some chains it is probabilistic: the more blocks built on top of a transaction, the less likely it is to be undone, but there is never an absolute guarantee. On others it is deterministic: after a specific step in the protocol, reversing the transaction is impossible without breaking the rules.

Bitcoin has probabilistic finality. A transaction with one confirmation could in theory be reversed by a competing chain, which is why exchanges typically wait for six confirmations, about an hour, for large deposits. Ethereum's proof of stake has deterministic finality: a block is finalized after two epochs, roughly 13 minutes, once two-thirds of validators have attested to it. Solana reaches finality in a few seconds.

Finality matters for anyone accepting payment. A merchant who ships goods before finality risks the payment being reversed. Bridges and exchanges set confirmation requirements based on each chain's finality properties, which is one reason deposits from different networks are credited at different speeds.