Merkle Tree

A Merkle tree, also called a hash tree, is a way of organising many pieces of data so that all of them can be summarised by one hash. Each transaction is hashed individually, then hashed together in pairs, then those results are paired again, level after level, until a single value remains at the top: the Merkle root. Bitcoin, Ethereum and most other blockchains store only that root in the block header instead of the full transaction list. The structure has two useful properties: changing any single transaction changes the root, and a light client can prove that one transaction belongs to a block by checking a short path of hashes rather than downloading everything.