yoctowatt (yW) The septillionth part [U.S.] or quintillionth part [U.K.] 10-24 [0.000 000 000 000 000 000 000 001] of a watt. Related 'metric' families: yotta; zetta; exa; peta; tera; giga; mega; kilo; hecto; deka; deci; centi; milli; micro; nano; pico; fe...