Source : Free On-Line Dictionary of Computing

inverted index
     
         A sequence of ({key}, pointer)
        pairs where each pointer points to a {record} in a {database}
        which contains the key value in some particular field.  The
        index is sorted on the key values to allow rapid searching for
        a particular key value, using e.g. {binary search}.  The index
        is "inverted" in the sense that the key value is used to find
        the record rather than the other way round.  For databases in
        which the records may be searched based on more than one
        field, multiple indices may be created that are sorted on
        those keys.
     
        An index may contain gaps to allow for new entries to be added
        in the correct sort order without always requiring the
        following entries to be shifted out of the way.
     
        (1995-02-08)