We say data, particularly time seres data is in the frequency domian when we represent it in terms of components at differet frequencies rather than as samples at different time points (time domain). The transformation between tome domain and frequency domain is typically acheievd using a Fourier transform and subsequent processing calledd Fourier analysis. Some types of processing are more easily acheived 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 spectorgram that shows moment to moment frequency analysis of a sound source.
Used on Chap. 6: pages 124, 125; Chap. 14: page 320
Also known as frequency space
Digitally produced spectrogram of a male voice saying `nineteenth century' (source: Wikipedia) -- a sectrogram is a form of Fourier transform
