(music) effects unit (effects pedal, etc.)

(biology) effector

factor (one of the elements, circumstances, or influences which contribute to produce a result)

either one of the two Kamikaze-class destroyers of the Imperial Japanese Navy (see Japanese destroyer Yūnagi on Wikipedia.Wikipedia) / a female given name / a place name

Morse code

a pendulum clock

(computing) firewall

(Anglicism, attributive) fourth

(Anglicism) force

the first of the 勅撰和歌集 (chokusen wakashū, “imperial waka anthologies”), compiled in 905-914 CE