Merkle root

The root is built recursively: each piece of data is hashed, then paired and hashed upward until a single node remains. Its key property is that any change in any leaf alters the root, thanks to the collision resistance of the underlying hash. That is why one compact value suffices to commit to large volumes of data and later validate individual Merkle proofs against it. The root is anchored in a small place, such as a block header, a contract or a certificate.

It commits to enormous sets with a single hash and then verifies the membership or integrity of any element efficiently, keeping only a compact anchor on-chain. It is the piece that gives meaning to the inseparable pair of Merkle root and Merkle proof.

In Bitcoin, each block header contains the Merkle root of the transactions, present since the genesis block of January 2009. In Ethereum, state, transactions and receipts are committed via Merkle-Patricia tree roots stored in the header, exposed as stateRoot, transactionsRoot and receiptsRoot. RFC 6962 (Certificate Transparency) also formalizes the tree structure and its root. Other real uses are the roots published in airdrop contracts, such as Uniswap's UNI, and in exchange proof of reserves.