Slack is the extra space on a hard drive that surrounds a file. An example of this is if a user were to write and save a three page Microsoft Word document and then the next day decide that only one page is necessary, delete the last two pages and save the document again. When erasing the now one page document with Blancco File Eraser, the missing second and third pages (which are no longer visible but still remain on the hard drive) will also be caught and overwritten.