What is a leaf node in a Merkle tree?

A leaf node is the entry point of a Merkle tree — it stores the cryptographic hash of a single data block, such as one transaction. To create a leaf node, a data block is run through a hash function such as SHA-256 or BLAKE2b, which produces a unique, fixed-length value that acts as a digital fingerprint of that data. Because even a tiny change to the original data produces a completely different hash, the leaf node makes tampering immediately detectable. For a beginner, this matters because it is the mechanism that allows a blockchain to prove — quickly and cheaply — that transaction data has not been altered.