What is Frequency?

The frequency $f$ of a vibration is the number of complete back-and-forth swings it makes each second. Its unit is the hertz ($\mathrm{Hz}$): one swing per second — honoring the physicist who first made and caught invisible waves. A guitar string trembling $110$ times a second vibrates at $110\,\mathrm{Hz}$; a mosquito’s wings, near $600\,\mathrm{Hz}$; the slow flap of a big flag, a few hertz.

The frequency of a periodic signal is the number of cycles per second:

Its unit, one cycle per second, is the hertz ($\mathrm{Hz}$), with the usual multiples $\mathrm{kHz}$, $\mathrm{MHz}$, $\mathrm{GHz}$.