When dealing with signals such as vibration signal, sometimes PSD comes into our attention. PSD was used to measure signal power content/energy variation in the frequency domain and it is measured using complex mathematical calculations and measured as frequency per unit mass.
Power
This term indicates the magnitude of power spectral density functions in respect to rms of the vibration signal. For PSD, the power is not affiliated with power measured in Volt or horsepower, but referring to the rms of any value is known as the power of that value.
Spectral
Spectral term here to indicate that PSD is a frequency-based function which represent the signal distribution on a frequency spectrum.
Density
Density refers to the PSD function magnitude which has been normalized to a bandwidth of 1Hz
Units
PSD unit can be g2/Hz or (ms-2)2 /Hz
References
https://www.safeloadtesting.com/power-spectral-density/
https://community.sw.siemens.com/s/article/what-is-a-power-spectral-density-psd