Power density
Beginner
How much heat a patch of chip gives off for its size. Too much in one spot and the chip overheats.
Novice
Power divided by area, in watts per square centimeter or per square millimeter. Cooling limits how high it can go.
Expert
Dennard scaling held it constant; once voltage stopped scaling, power density rose with transistor density at a given activity and frequency. It is what a power budget really limits, and why clock rates stalled and dark silicon appeared.