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.