1550s, "state of being crowded" (now obsolete); 1640s, "fact of occurring often;" from Latin frequentia "an assembling in great numbers, a crowding; crowd, multitude, throng," from frequentem (see frequent). Sense in physics, "rate of recurrence," especially of a vibration, is from 1831. In radio electronics, frequency modulation (1922, abbreviated F.M.) as a system of broadcasting is distinguished from amplitude modulation (or A.M.).