misprediction penalty
The cycles lost each time a branch guess turns out wrong; on today's big cores roughly 15 to 20.
The misprediction penalty is the cycles between fetching down the wrong path and restarting fetch on the right one, about the pipeline depth up to branch resolution (around 15-20 cycles on current high-performance cores); total cost = mispredictions × penalty.
Examples
With 20% branches, 5% of them mispredicted, and a 15-cycle penalty, CPI rises by 0.2 × 0.05 × 15 = 0.15.