Finality

The point after which a blockchain transaction cannot be reversed without breaking the network's security guarantees.

• Bitcoin's finality is probabilistic: each new block makes reversal exponentially harder, and 6 blocks, about an hour, is the usual convention.
• Ethereum's is economic: after about 13 minutes, reversing a block would require destroying at least a third of all staked ETH.
• Exchanges and bridges wait for finality before crediting deposits; otherwise a reorganised chain would allow double spending.
• Faster finality needs more messages between validators; Ethereum researchers are working toward finality within a single 12-second slot.