Finality in a Proof-of-Stake (PoS)

Different Proof-of-Stake (PoS) based blockchains have different designs for attaining finality. Attaining finality in PoS systems is a major discussion in today's PoS blockchain based systems.

Ethereum uses the Casper protocol to achieve finality.

Casper manages finality by using "checkpoint" blocks. The first block in each epoch is a checkpoint. Validators vote for pairs of checkpoints that it considers to be valid. If a pair of checkpoints attracts votes representing at least two-thirds of the total staked ETH, the checkpoints are upgraded. The more recent of the two (target) becomes "justified". The earlier of the two is already justified because it was the "target" in the previous epoch. Now it is upgraded to "finalized". See more here: https://ethereum.org/en/developers/docs/consensus-mechanisms/pos/