Finality
In simple terms
Finality is when a transaction is genuinely done. Some networks make this a definite moment; others make it a matter of confidence that grows with each block, and never quite reaches absolute certainty.
Definition
The point at which a transaction is settled and cannot be reversed by a reorganisation of the chain.
In depth
Probabilistic finality, used by proof-of-work chains, gives a reversal probability that decays with confirmation depth but is never zero. Deterministic finality, provided by BFT-style consensus and by proof-of-stake finality gadgets, marks a checkpoint that honest validators will not revert, backed by slashing conditions that make equivocation costly. Layer 2 systems add their own layer: an optimistic rollup transaction is soft-confirmed instantly but only final on the settlement layer after the challenge window elapses, while a validity rollup is final once its proof is verified.
How does Finality work?
Under probabilistic finality, each new block makes rewriting history more expensive, and services pick a depth where that cost exceeds any plausible gain. Under deterministic finality, validators vote in rounds and a checkpoint becomes final once a supermajority has attested, after which reverting it would require a large fraction of them to be slashed. Rollups inherit their settlement layer's finality and add their own delay, which is why a withdrawal from an optimistic rollup can take days.
An example
A transfer on an optimistic rollup shows as complete in the app within seconds, and moving those funds back to the settlement layer takes an illustrative seven-day challenge window. Nothing is wrong during that week; the delay is the period in which anyone may submit a fraud proof.
Figures are illustrative only.
What beginners get wrong
- Reading "instant" in an application as final. Fast user-facing confirmation and settlement finality are different things.
- Expecting a rollup withdrawal to be quick because the deposit was. The two directions are not symmetric.
- Assuming finality is one universal standard. Every network defines it differently, and the differences are large.
Related terms
Part of
How does a blockchain transaction actually work? — the subject page for blockchain mechanics, with all 15 of its definitions in one place.
Educational only — not financial advice.