Frequency
Etymology
From Latin "frequentia" (a crowding, a throng), from "frequens" (crowded, repeated). The idea of things packed close together in space shifted to things packed close together in time.
noun · /FREE-kwen-see/
How often something repeats per unit of time — for a wave, the number of cycles that pass each second.
What it names
The rate of repetition, the handle that separates a low hum from a high whine, or an AM station from an FM one.
Example
Raising a sound’s frequency raises its pitch; doubling it jumps the note up an octave.
Related idea
Frequency counts cycles per second; wavelength measures their spacing. Higher frequency means shorter wavelength, so a treble note and a soprano share high frequency but tiny wavelengths.