Finality
The point at which a transaction becomes irreversible — instant in some systems, probabilistic and gradual in others.
Finality is the guarantee that a confirmed transaction can no longer be reversed or altered. It comes in two flavors: probabilistic finality (Bitcoin’s — reversal becomes exponentially unlikely as blocks pile on, but is never mathematically impossible) and deterministic finality (BFT systems — a transaction is final the instant it’s committed).
Finality matters practically: it’s why exchanges wait for multiple block confirmations before crediting deposits, and why a chain’s finality time affects its usability for payments. It’s also central to Layer 2 design — how and when an L2’s transactions achieve finality on the base layer determines their real security. Faster finality is convenient; the guarantees behind it are what actually matter.