Clock uncertainty
Beginner
A safety margin for the clock tick wobbling a little from cycle to cycle.
Novice
A safety margin, set in the timing constraints, that is taken away from the time available for setup and added to the hold requirement. It covers jitter (the tick wobbling slightly from one cycle to the next) and, before the clock tree exists, the skew the tree is expected to add.
Expert
Before CTS: jitter + skew estimate + margin. After CTS the skew part is removed, because the timer now computes real skew. Setup and hold values are set separately, and inter-clock uncertainty covers paths between two clocks.
Explained in Clock tree synthesis (Design Flow).