The trillionth [U.S.] and billionth part [U.K.], 10^-12 [0.000 000 000 001] of a watt.

In biomedicine, one pW is the average power consumption of a human cell; while 2.5 pW is the sound intensity per square centimeter for the average human threshold of hearing at 1000 Hz; and 150 pW is the power that enters a human eye from a 100 watt lamp one km away.