Frequency
The number of times a sound wave repeats every second (Hz).

Frequency represents the number of times a sound wave repeats every second. That number is cited as "Hertz" -- thus "20Hz" denotes twenty repetitions of the sound wave per second.

We hear this repetition as a "pitched" sound. Musical pitch is produced from regularly-repeating (that is, "periodic") sound waves (or waveforms).

Frequency is the scientific term for what musicians refer to as pitch. Pitch and frequency are effectively synonymous. However, we also speak of a frequency as "having a pitch." For human beings to hear the pitch of a frequency, it generally must lie within what is called the "auditory range" -- that is, between 20Hz and 20,000 Hz. Frequencies lying outside that range are generally inaudible to the human ear.

Fig 1: a periodic waveform with a period indicated.