Frequency domain is a term used to describe the analysis of mathematical functions with respect to frequency.

Speaking non-technically, a time domain graph shows how a signal changes over time, whereas a frequency domain graph shows how much of the signal lies within each given frequency band over a range of frequencies. A frequency domain representation also includes information on the phase shift that must be applied to each frequency in order to be able to recombine the frequency components to recover the original time signal.

If a signal is known in the time domain, a frequency domain graph can be constructed using a technique known as a Fourier transform. This technique relies on Fourier series. The result is a frequency spectrum.

In many applications, phase information is not important. By discarding the phase information it is possible to simplify the information in a frequency domain representation to generate a frequency power spectrum. A spectrum analyser on a stereo is an example of a frequency power spectrum.

A biological system that appears to operate primarily in the frequency domain is the auditory system, in which the basilar membrane of the inner ear is able to perform a power spectrum decomposition of incoming sound waves. The result, is that we are able to hear a collection of different frequencies played together as a collection of separate notes, rather than simply a complicated noise.