The Free On-line Dictionary of Computing (30 December 2018):

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)