The basic definition of power density is the rated (or nominal) output power of a device divided by the volume (volume = x · y · z) occupied by the device. Common units of measurement for power density can be watts per cubic meter (W/m3) or watts per cubic inch (W/in.3).
Power spectral density (PSD) is the measure of a signal's power content versus frequency. PSD gives designers an idea of how the power of a signal is distributed over frequency. PSD is usually chosen to characterize random broadband signals.