Finality describes when, or with what confidence, a blockchain's recorded history can be treated as settled. It depends on the consensus mechanism and its assumptions. A transaction appearing in a block is not necessarily at the network's strongest settlement stage.
Different forms of assurance
Bitcoin offers probabilistic settlement: additional blocks generally make replacing an earlier transaction less likely, under assumptions about competing hash power. There is no block count that creates an unconditional mathematical guarantee. Some proof-of-stake systems instead use validator voting to finalize checkpoints under explicit fault thresholds.
Ethereum's checkpoint process
Ethereum distinguishes the current chain head from justified and finalized checkpoints. Finalization relies on supermajority stake votes and protocol rules. Serious consensus failures can violate those assumptions, while insufficient participation can delay finalization even when new blocks continue to appear.
Separate technical and service outcomes
A finalized transaction can still have executed unsuccessfully, so finality is not proof of a successful transfer. Exchanges may also apply additional crediting requirements. On a rollup, an early sequencer receipt, settlement on the base layer and availability of a withdrawal can occur at different times. The relevant question is which guarantee has been reached for the specific operation.