Frequency is the number of cycles per second of a waveform, measured in hertz (Hz). In other words, it is how often a sound wave repeats itself. Frequency is related to the pitch of a sound, but is not the same thing. Whereas frequency is a purely physical quantity, pitch is a psychoacoustic property which depends not only ton he frequency or combination of frequencies of a sound but also how that sound is perceived by the listener.
Frequency of musical instruments
In the case of musical instruments, smaller ones tend to produce more rapid vibrations and larger ones slower vibrations: thus, for example, the oboe produces generally higher frequencies than does the bassoon, a violin than a cello, a stopped string than the same string left open, and a woman's voice than a man's voice. But other factors enter into the control and production of different frequencies. In particular, the mass (the thinner strings of a violin vibrate more quickly than the thicker ones and so produce higher frequencies) and tension (a guitar string tightened but turning the tuning peg produces higher frequencies).
A tuning fork produces sound of just one frequency. However, the sounds produced by musical instruments always contain a mixture of frequencies, even if just a single notes is played. These frequencies include the fundamental and various harmonics, overtones, and upper partials, which affect not only the perceived pitch but also the timbre, or "color", of the sound.
See also frequency response.