Proof of History, PoH, is a cryptographic clock that orders events without nodes arguing over timestamps. Each event is hashed with the previous hash, forming a sequence anyone can verify. PoH does not replace Proof of Stake. Consensus still agrees on blocks. PoH gives that consensus a pre-ordered log.
Solana is the main chain using it. Because order is already in the hash chain, validators spend less chatter agreeing on sequence, which helps the network process thousands of transactions per second. A leader produces the hash chain, which is a centralization point. Anyone can still verify the chain, and leadership rotates.
It provides an immutable ordering that consensus protocols reference when determining transaction sequence. Solana is the most prominent blockchain using PoH. The tradeoff is that PoH requires a single leader node to produce the hash chain, which introduces a centralization point.
Solana's whitepaper describes Proof of History as a verifiable delay so nodes agree on time without waiting on a clock message.
Proof of History (PoH)
A verifiable clock built into the blockchain via sequential hashing.