Clock skew
Beginner
The gap in time between the same clock tick reaching two different parts of the chip.
Novice
The difference between the times the same clock tick reaches two flip-flops. On this page, skew = arrival at the receiving (capture) flip-flop minus arrival at the sending (launch) one, so positive skew gives the data traveling between them extra time.
Expert
Global skew is the latest minus the earliest arrival over all sinks. Local skew is measured only between flip-flops joined by a data path, and only local skew decides whether timing passes. Textbooks and tools disagree on the sign, so check which arrival is subtracted.
Explained in Clock tree synthesis (Design Flow).
See also: Useful skew, Insertion delay (clock latency).