Slack
Beginner
How much spare time a signal has before it would arrive too late. Positive is good; negative means it is late.
Novice
The spare time on a timing path: when a signal is required at its destination minus when it actually arrives. Negative slack means it arrives too late and the path fails.
Expert
Required time minus arrival time at a path’s endpoint, for a setup or hold check. The worst value in a group of paths is WNS (worst negative slack) and the sum of the negative values is TNS (total negative slack). Synthesis computes it with a perfect clock and guessed wires, so teams aim for some positive margin.
Explained in Logic synthesis (Design Flow).
See also: Critical path, SDC (timing constraints).