Hash (Cryptographic).

A one-way math function that turns any input — a word, a file, a block of transactions — into a fixed-length fingerprint. The same input always produces the same hash; change a single character and the hash looks totally different. The honest caveat: hashes are how blockchains 'lock' history together, but they aren't magic — they only work because finding two inputs with the same hash is, in practice, computationally infeasible.