Jim Benson gives an interesting definition of slack in Flow, Cadence, Slack, and Stress in his Lean Muppets series (fun for the videos!)
Slack: The gaps between work that make flow possible and define cadence
This is an interesting look at the importance of slack, since most people think it is a bad thing for people or processes. But without the space between activities or the opportunity to breath, minor variations and hiccups get blown all out of proportion.