frequency (n.) Look up frequency at Dictionary.com
1640s, "fact of occurring often," from Latin frequentia "a crowding, crowd," from frequentem (see frequent).

Earlier it had been used in a now-obsolete sense of "state of being crowded" (mid-16c.); 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.).