Proof of History is a consensus algorithm for verifying blockchain order and the passage of time. It is a component of the Solana network alongside Solana's PoS consensus. A PoH generator sequentially hashes data where the output of a hashing round is the input to the next hashing round. Appending data into this hashing sequence timestamps the data. PoH represents the verifiable passage of time through an hourglass composed of the "poh_service.rs" source code from the Solana protocol.