Crypto glossary
Proof of history (PoH)
Proof of history (PoH) is a verifiable clock built from sequential hashing that orders events before consensus, popularised by Solana to speed up its chain.
PoH runs a hash function, SHA-256 on Solana, over and over, feeding each output into the next. Because each step depends on the previous one, the sequence cannot be computed in parallel, so its length proves that time has passed. Inserting a transaction's hash into the sequence stamps it with a position that anyone can verify later.
Solana's founder Anatoly Yakovenko introduced the idea so that validators would share a common sense of time without exchanging messages to agree on it. The leader for each slot produces the PoH sequence and the transactions inside it, and other validators can verify the ordering quickly, which helps the network target slots of around 400 milliseconds.
PoH is not a consensus mechanism on its own and does not make the chain Sybil-resistant; Solana still relies on proof of stake and a BFT-style voting protocol called Tower BFT for security and finality. Critics also note that the speed depends on demanding validator hardware, which raises the cost of running a node.
Example
A Solana leader receives a swap, hashes it into its running PoH sequence at a specific tick, and every validator can check exactly where that swap sits relative to every other transaction in the slot.