hash function

one-way function (not reversible) that changes input data in to a unique digital code (also used in information security as a "digital signature" that identifies the sender and verifies the contents of the message)

A hash function is a reproducible method of turning some kind of data into a (relatively) small number that may serve as a digital "fingerprint" of the data. The algorithm "chops and mixes" (i.e., substitutes or transposes) the data to create such fingerprints. The fingerprints are called hash sums, hash values, hash codes or simply hashes. (Note that hashes can also mean the hash functions.) Hash sums are commonly used as indices into hash tables or hash files.  Cryptographic hash functions are used for various purposes in information security applications.
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<programming> A hash coding function which assigns a data item distinguished by some "key" into one of a number of possible "hash buckets" in a hash table. The hash function is usually combined with another more precise function.
For example a program might take a string of letters and put it in one of twenty six lists depending on its first letter. Ideally, a hash function should distribute items evenly between the buckets to reduce the number of hash collisions. If, for example, the strings were names beginning with "Mr.", "Miss" or "Mrs." then taking the first letter would be a very poor hash function because all names would hash the same.
(1997-08-03)