Power Spectral Density
A description of how a signal's average power is distributed across frequency.
Signal Processing Context
Power spectral density describes how signal power is distributed over frequency. Estimators such as Welch's method trade frequency resolution for lower variance through windowing, segmentation, and averaging, which is useful for noise-floor and band-power analysis.
Measurement Boundary
The squared magnitude of a single FFT is not automatically a correctly normalized PSD estimate. Sampling frequency, window energy, segment length, and one-sided or two-sided conventions determine units and comparability.