Proof-of-work chains offer probabilistic finality: reversal never becomes impossible, only absurdly expensive. Many proof-of-stake chains offer economic finality, where reverting a finalised block would require slashing a large fraction of total stake.
Finality is the number that matters for settlement, bridging and arbitrage. A bridge that releases funds before source-chain finality can be drained by a deliberate chain-reorg.
Example: an Ethereum-style chain finalises roughly every two epochs, about 13 minutes. A ten-minute-block chain reaches comparable practical safety after about six blocks, an hour. Plan collateral transfers around whichever figure applies, not around the "sent" notification in your wallet.
Related: confirmation, chain-reorg, epoch, bridge