The Power Spectral Density (PSD) is the distribution of signal power over frequencies.

It is often estimated by sampling a signal for a finite amount of time, taking the Discrete Fourier Transform (DFT) of the resulting sample vector, and considering the magnitude square of the resulting DFT bins.

Notice that this is, for a random signal, a property of the stochastic process underneath. See: Wiener-Chintschin theorem.