Source : Free On-Line Dictionary of Computing

one-way hash function
     
         (Or "message digest function") A {one-way
        function} which takes a variable-length message and produces a
        fixed-length hash.  Given the hash it is computationally
        infeasible to find a message with that hash; in fact one can't
        determine any usable information about a message with that
        hash, not even a single bit.  For some one-way hash functions
        it's also computationally impossible to determine two messages
        which produce the same hash.
     
        A one-way hash function can be private or public, just like an
        {encryption} function.  {MD5}, {SHA} and {Snefru} are examples of
        public one-way hash functions.
     
        A public one-way hash function can be used to speed up a
        public-key {digital signature} system.  Rather than sign a
        long message, which can take a long time, compute the one-way
        hash of the message, and sign the hash.
     
        {sci.crypt FAQ
        (ftp://src.doc.ic.ac.uk/usenet/usenet-by-group/sci.crypt/)}.
     
        (2001-05-10)