Hash
A fixed-length digital fingerprint of any data — change one character and the fingerprint changes completely.
A hash is the output of a one-way function that converts any input into a fixed-length string. The same input always yields the same hash; a one-character change produces an unrecognizably different one; and reversing the process is computationally infeasible. Bitcoin uses SHA-256, producing 64-character hexadecimal fingerprints.
Hashes are blockchain’s connective tissue: every block contains its predecessor’s hash (forming the chain), transaction IDs are hashes, addresses derive from hashed public keys, and mining is a race to find a hash below a target. “Immutability” is really just hashes making any historical edit instantly visible.