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.