Finality
The point at which a recorded transaction can no longer be undone by the network reorganizing its recent history.
Proof-of-work chains offer probabilistic finality: each additional block makes reversal exponentially more expensive but never mathematically impossible, which is why services wait for a set number of confirmations before crediting a deposit. Byzantine fault tolerant proof-of-stake designs offer explicit finality instead, where a supermajority vote marks a block as finalized and reversing it would require destroying a large fraction of the total stake, commonly at least one third, which is called economic finality. Tendermint-style chains finalize each block within a single voting round, while Ethereum finalizes checkpoints roughly every two epochs. Readers meet finality in exchange crediting policies, in bridge designs that must decide when a source-chain event is safe to act on, and in incident reports when a network stops finalizing.
In practice
Ethereum finalizes checkpoint blocks after two epochs of attestations, and a network that stops finalizing while still producing blocks is treated as being in a degraded state.
The common misunderstanding
Confirmed and final are not the same thing; a confirmed transaction on a probabilistic chain can still be undone by a deep reorganization.