hole

hole n.  A region in an otherwise {flat} entity which is
   not actually present.  For example, some UNIX filesystems can store
   large files with holes so that unused regions of the file are never
   actually stored on disk.  (In techspeak, these are referred to as
   `sparse' files.)  As another example, the region of memory in IBM
   PCs reserved for memory-mapped I/O devices which may not actually
   be present is called `the I/O hole', since memory-management
   systems must skip over this area when filling user requests for
   memory.