Source : Free On-Line Dictionary of Computing

backing store
     
        1.  Computer memory, usually {magnetic disks},
        storing data and programs.  Sections of this information can
        then be copied into the main memory ({RAM}) for processing.
        Backing store is cheaper but RAM is faster.  Such a hierarchy
        of memory devices allows a trade-off between performance and
        cost.
     
        2.  Character storage in memory or on disk, as opposed
        to displayed or printed characters.  This distinction is
        important where the visual ordering of characters differs from
        the order in which they are stored, e.g. bidirectional or
        non-spacing layout.
     
        In a {Unicode} encoding, text is stored in sequential order in
        the backing store.  Logical or backing store order corresponds
        to the order in which text is typed on the keyboard (after
        corrections such as insertions, deletions, and overtyping).  A
        text rendering process converts Unicode text in the backing
        store to readable text.
     
        ["The Unicode Standard: Worldwide Character Encoding", Version
        1.0, Vol. 1. Addison-Wesley, 1991].
     
        (2001-02-25)