Hash

A hash is the fixed-length fingerprint that a cryptographic hash function produces from any input — a file, a transaction, or an entire block. The same input always yields the same hash, while changing a single character produces a completely different one, and there is no practical way to work backwards from the hash to the original data. Blockchains use hashes everywhere: a transaction hash (TXID) identifies a payment so you can look it up in a block explorer, and each block header contains the hash of its parent, which is what chains the history together. A hash is not secret and can be shared, but check it character by character when verifying an address, file or transaction.