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

flat address space

    The memory architecture in which any memory
   location can be selected from a single contiguous block by a
   single integer offset.

   Almost all popular processors have a flat address space, but
   the Intel x86 family has a segmented address space.  A
   flat address space greatly simplifies programming because of
   the simple correspondence between addresses (pointers) and
   integers.

   (1996-09-10)