slack

slack n.  1. Space allocated to a disk file but not actually
   used to store useful information.  The techspeak equivalent is
   `internal fragmentation'.  Antonym: {hole}.  2. In the theology
   of the {Church of the SubGenius}, a mystical substance or
   quality that is the prerequisite of all human happiness.

   Since UNIX files are stored compactly, except for the unavoidable
   wastage in the last block or fragment, it might be said that "Unix
   has no slack".  See {ha ha only serious}.