What are a Merkle tree and its Merkle root?

A Merkle tree hashes transactions in pairs, then hashes those hashes, and so on up to a single top hash — the Merkle root — that fingerprints every transaction at once.

The block header commits only to the root, so a lightweight client can prove one transaction is included with a small "Merkle branch," and old spent branches can be pruned without changing the root.

Go deeper:

• Merkle tree — Wikipedia — diagrams the binary hash tree from leaves up to the root.