What is "hash"

A hash function is any well-defined procedure or mathematical function that converts a large amount, possibly of variable data size, to a small data, usually a single integer that can serve as an index for an associative matrix. The values returned by a hash function are called hash values, hash codes, hash sums, checksums or simply hashes.

An cryptographically strong hash function has two characteristics: it is irreversible and minimizes collisions. Irreversibility means that the original data cannot be reconstructed from its hash. A collision occurs when two different datasets generate the same hash.

Hash functions are related to (and often confused with) checksums, check digits, fingerprints, randomization functions, and error correction codes. Although these concepts overlap to some extent, each has its own needs and uses and is designed and optimised differently.