A Merkle leaf is the bottom-most node of a Merkle tree, which is a binary and hierarchical data structure that efficiently organises and verifies data using cryptographic hashes. A Merkle leaf contains the hash value of a single transaction data in the context of blockchain.
Each node in a Merkle tree is labelled with a cryptographic hash of the data block, which is used to ensure the data integrity and efficient verification of large datasets, such as transactions in a block. In a Merkle tree, the tree starts with leaf nodes (Merkle leaves), moving up where pairs of nodes are hashed together to form their parent node until reaching the Merkle root at the top.
Instead of having to verify all transactions in a block, validators can use a process called a Merkle proof (also known as Merkle path) to verify that a particular transaction (Merkle leaf) belongs to the block by checking only a small subset of hashes (the path up the tree to the root).
By using a Merkle tree, blockchains can efficiently store and verify large amounts of transaction data. A Merkle leaf ensures that, even if a small part of the data is modified, the cryptographic hash will change, alerting the system to tampering.
