Source : Free On-Line Dictionary of Computing

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)