Finality

Finality is the point at which a blockchain transaction can no longer be reversed or changed. Until a transaction is final, there is a chance it could be dropped if the chain reorganises.

Proof-of-work chains such as Bitcoin have probabilistic finality: each new block on top of a transaction makes reversal exponentially more expensive, but never strictly impossible. That is why services wait for several confirmations. Many proof-of-stake chains offer economic finality. On Ethereum, blocks are finalised after two epochs, roughly 13 minutes, and reverting a finalised block would require at least a third of all staked ETH to be destroyed by slashing. Some chains, such as those using Tendermint-style consensus, finalise blocks within seconds.

For users, finality explains why deposit times differ between networks and why bridges and exchanges are cautious. Layer 2 rollups add another layer: a transaction may be confirmed quickly on the rollup but only fully settled once it is final on Ethereum.