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.

