Hash

A hash is a short string of characters produced by running any data through a fixed one-way calculation, giving a kind of digital fingerprint for that exact data. Change a single letter of the input and the hash changes completely, and you cannot work backwards from the hash to the original.

It is like a tamper-check on a sealed document: the same contents give the same fingerprint, and any edit shows up at once. Hashes matter because they let you verify that a file, a password check or a record has not been altered, quickly and without storing the sensitive original.