Spectral Density is a term used in signal processing and Time Series Analysis and similar terms are used to describe the distribution of Electromagnetic Radiation or other Frequency-related phenomena.
Power Spectral Density (PSD) is the distribution of power as described by the Fourier Transform of the auto-correlation function of a signal. It shows the strength of the variations in power as a function of frequency.
Energy Spectral Density (ESD) is similar, taken over a specific interval of time.
The terms energy and power are "borrowed", used because one is an amount and the other a rate of the same concept over time, analogous to how the terms are used for physical energy.
Magnetic Energy Spectrum