Non-default routing rule (NDR)
Beginner
Special, roomier wiring rules used for the clock so it stays fast and quiet.
Novice
A wiring rule that overrides the normal wire width and spacing for chosen wires. Clock wires often get double spacing, so neighboring wires disturb them less, and sometimes double width, which lowers their resistance.
Expert
Double spacing cuts coupling capacitance and so crosstalk-induced delay changes; double width cuts resistance and helps electromigration. Usually applied to the trunk and upper levels; leaf nets keep default rules to save routing tracks.
Explained in Clock tree synthesis (Design Flow).