Eilene Marie Slack Galloway has written:

'Convention on registration of objects launched into outer space' -- subject(s): Convention on registration of objects launched into outer space

Basically it's just that, slack. It is a lack of something, or in this case memory. If you have slack on your PC your files have bits in between them, taking up space that is not being used. It is the difference between physical size and logical size. File slack is also related to computer fragmentation where storage is used inefficiently. I believe on PCs you have an option of defragmenting your hard drive to relieve space.

The wasted space is called slack.

organisational slack