In cryptography and computer science, a hash tree or Merkle tree is a tree in which every leaf node is labelled with the cryptographic hash of a data block, and every non-leaf node is labelled with the cryptographic hash of the labels of its child nodes.
Hash trees allow efficient and secure verification of the contents of large data structures.
In bitcoin’s blockchain, a block of transactions is run through an algorithm to generate a , which is a string of numbers and letters that can be used to verify that a given set of data is the same as the original set of transactions, but not to obtain the original set of transactions.
Visualized, this structure resembles a tree. The Merkle tree is useful because it allows users to verify a specific transaction without downloading the whole blockchain (over 350 gigabytes at the end of June 2021).