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For a given signal, the power spectrum gives a plot of the portion of a signal's power (energy per unit time) falling within given frequency bins. The most common way of generating a power spectrum is by using a Fourier Transform, but other techniques such as the Maximum Entropy Method can also be used.
References
Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T.  ``Power Spectra Estimation Using the FFT''
  and ``Power Spectrum Estimation by the Maximum Entropy (All Poles) Method.''  §13.4 and 13.7 in
  Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed.  Cambridge, England:
  Cambridge University Press, pp. 542-551 and 565-569, 1992.