Glossary
Finality
The point at which a confirmed transaction can no longer be reverted. Probabilistic on PoW chains, explicit on most modern PoS chains.
On Bitcoin, finality is probabilistic: a one-block-old transaction could plausibly be reorged, but a six-block-old one effectively cannot without re-mining the chain — which would cost more than any transaction is worth.
On Ethereum PoS, finality is explicit: a block is "finalized" once two-thirds of stake have attested to it across two consecutive epochs. Reverting it requires the same supermajority to be slashed. Other PoS chains (Cosmos-style BFT, Solana) have similar guarantees with different specifics.