(1) A function that converts a message into a fixed string of digits.
(2) An algorithm that converts processing data into a string of bits; known as a hash value or a message digest.
(3) In cryptography, pertaining to a type of algorithm that can be applied to a buffer of data to produce a fixed string of digits. Cryptographic programmes often apply one-way hash algorithms against a set of data and often compare the results with previously generated hash values.