Finality is the point at which a transaction on a blockchain is considered permanent and cannot be reversed. It sounds like a yes/no question but it actually comes in several flavours, which is one of the reasons people writing about different chains sometimes talk past each other on this topic.
Probabilistic finality is what Bitcoin has. A transaction is never strictly final — in principle, a long enough chain reorganisation could always unwind it — but the probability of that happening drops exponentially with each subsequent block. After one confirmation the transaction is pretty safe. After six confirmations (about an hour) it is commercially safe for most purposes. After a day it is safe against everything short of a state-level attacker. You do not get a specific moment of finality; you get an ever-decreasing probability of reversal.
Absolute (or deterministic) finality is what Cosmos, Algorand, Polkadot and other Tendermint-derived chains offer. A block is either finalised or it is not, and finalised blocks can never be reverted as long as the underlying BFT assumption (less than one-third adversarial stake) holds. Finality is usually reached within a few seconds of the block being produced. The trade-off is that during network partitions or validator failures the chain can halt rather than fork — it prefers safety to liveness.
Economic finality is Ethereum’s post-Merge concept. Transactions are probabilistically final at first (within a few seconds of inclusion in a block), and then reach full finality after two more “epochs” of attestations — roughly 13 minutes under normal operation. Reversing a finalised Ethereum block would require a supermajority of stakers to be slashed, which costs billions of dollars in aggregate. It is not mathematically impossible, but it is so expensive that it is considered effectively final.
Why This Actually Matters
Finality determines how long you have to wait before it is safe to treat a transaction as done. For small payments in stablecoins, you can usually act within seconds. For large OTC trades or bridge transfers, you want to wait for deep finality on both sides before you release anything irreversible. Bridge hacks have sometimes exploited the gap between “the transaction appears confirmed” and “the transaction cannot be unwound” — a chain reorganisation after the bridge releases the destination-side tokens leaves the bridge short.
The practical implication is that you should know which finality model the chain you are using has. “This transaction is confirmed” on Ethereum means one thing if you wait 13 minutes and something weaker if you act immediately. “This transaction is confirmed” on Bitcoin means very different things at one confirmation versus six. And smaller chains with lower hash rates or smaller validator sets tend to have much weaker real-world finality guarantees than the numbers suggest, because the cost of attacking them is actually achievable.