Power Spectrum
The [see page 11, spectrum] showing the energy present at each frequency in a signal.
Power (at a given frequency) is the sum of the squares of the signals real and imaginary (eg. negative frequencies) components at that frequency.
The phase relationship is lost during calculation of the power spectrum. Meaning you can't reconstruct the original signal from a power spectrum. You'll either need both the real and imaginary parts of the spectrum (seperately) or know the phase relationship between the power spectrum to reconstruct the original signal.
A signals spectrum is fully specified by the magnitude and phase at each frequency. This is the complex spectrum.