frequency domain

We say data, particularly time seres data is in the frequency domian when we represent it in terms of components at different frequencies rather than as samples at different time points (time domain). The transformation between time domain and frequency domain is typically achieved using a Fourier transform and subsequent processing called Fourier analysis. Some types of processing are more easily achieved in frequency domain, for example boosting the volume of certain frequencies. Often this transformation is performed on windows of a the time series rather than the whole time series, for example this is the case in a spectrogram that shows moment to moment frequency analysis of a sound source.