Activity factor (α)
Beginner
How often, on average, a part of the chip switches each clock tick.
Novice
How often, per clock tick, a signal charges up (goes from 0 to 1). The clock does this every tick (); a signal that flips every tick does it every other tick (); typical logic signals average around 0.1.
Expert
Depends on the workload, and is the least certain input at the spec stage. Early estimates come from the previous chip’s measurements, or from simulating representative workloads on the design code and feeding the recorded switching to a power-estimation tool.