Literally the number of times something occurs per unit of time. In the audio world the frequency of sound vibrations are directly related to what we hear as pitch, though the relationship is NOT linear. It is also inversely related to wavelength. We use the word frequency, and the values associated with it, as an objective way to speak about sound characteristics. Saying a unit has a frequency response of 20 Hz to 20kHz is much more accepted than specifying the response in terms of pitch.