hole

   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.

[glossary]
[Reference(s) to this entry by made by: {slack}]