A Merkle tree, also known as a hash tree, is a type of data structure that is used in cryptography and blockchain technology. It is a tree-like data structure that allows for the efficient and secure verification of large amounts of data. A Merkle tree is constructed by taking a group of data items and calculating the hash, or the "fingerprint", of each item. These hashes are then grouped together to form new hashes, and this process is repeated until there is only one hash left, known as the root hash. This root hash serves as a unique "fingerprint" for the entire data set, and it can be used to verify the integrity of the data. Merkle trees are used in many different applications, including blockchain technology and file integrity verification.