Source : Free On-Line Dictionary of Computing

hash coding
     
         (Or "hashing") A scheme for providing
        rapid access to data items which are distinguished by some
        {key}.  Each data item to be stored is associated with a key,
        e.g. the name of a person.  A {hash function} is applied to
        the item's key and the resulting hash value is used as an
        index to select one of a number of "hash buckets" in a hash
        table.  The table contains pointers to the original items.
     
        If, when adding a new item, the hash table already has an
        entry at the indicated location then that entry's key must be
        compared with the given key to see if it is the same.  If two
        items' keys hash to the same value (a "{hash collision}") then
        some alternative location is used (e.g. the next free location
        cyclically following the indicated one).  For best
        performance, the table size and {hash function} must be
        tailored to the number of entries and range of keys to be
        used.  The hash function usually depends on the table size so
        if the table needs to be enlarged it must usually be
        completely rebuilt.
     
        When you look up a name in the phone book (for example), you
        typically hash it by extracting its first letter; the hash
        buckets are the alphabetically ordered letter sections.
     
        See also: {btree}, {checksum}, {CRC}, {pseudorandom number},
        {random}, {random number}, {soundex}.
     
        (1997-08-03)