The power in a data series as a function of frequency. This is a quantity that describes the amplitude of sinusoidal structure in our data. It is a function of “fluctuation frequency”, or inverse time. That is, if you have a sinusoid with a certain period/frequency, each value of the power spectrum describes the amplitudes of sinusoids in the data with that exact period/frequency. As usual, one can talk about a power spectrum in other quantities, such as inverse length (see Electron-density Wavenumber Spectrum).