Finality
The point at which a transaction is considered irreversibly confirmed and cannot be altered or reversed.
Finality describes when a blockchain transaction is considered permanent. Different chains achieve finality in different ways and at different speeds. Bitcoin's finality is probabilistic — the more blocks built on top of a transaction, the harder it is to reverse, but it is never mathematically impossible. Six confirmations (~60 minutes) is the conventional standard for high-value transfers.
Ethereum's Proof of Stake provides economic finality: once a transaction is included in a finalised checkpoint (approximately every 12.8 minutes, or two epochs), it would require an attacker to burn at least one-third of all staked ETH to reverse it. BFT-based chains like Tendermint achieve deterministic finality — once a block is confirmed, it cannot be reversed under any circumstances.
Layer 2 networks add complexity: a transaction on Arbitrum has 'soft' finality from the sequencer immediately, but 'hard' finality only once the data is settled on Ethereum — which for Optimistic Rollups includes a 7-day challenge window. ZK-Rollups achieve hard finality as soon as the validity proof is verified on L1, typically within minutes.