Proof of History
A timestamping technique used by Solana that creates a verifiable sequence of events before validators vote on them.
A leader runs a continuous chain of SHA-256 hashes where each output can only be produced after the previous one, so the length of the chain is evidence that time passed. Transactions and events are inserted into that sequence, which means validators do not have to exchange messages to agree on ordering before they can vote, removing a large part of the communication cost of consensus. Solana pairs this with a stake-weighted voting protocol that decides which fork is canonical, so proof of history handles ordering while consensus handles agreement. Readers meet the term mainly in explanations of why Solana targets very short block times.
In practice
Because the hash sequence is sequential by construction, any observer can verify that events were recorded in a particular order without trusting the leader's clock.
The common misunderstanding
Proof of history is not a consensus mechanism on its own; it orders events, while a separate stake-weighted vote decides which chain is canonical.