A specific manifestation of the Wigner distribution (also the Wigner-Ville distribution) is the frequency vs. time dependence of some time dependent signal. Specifically, it is the Fourier transform of the autocorrelation of the time domain signal -- roughly what you'd get by lining up the Fourier transforms of various segments of the time domain signal (technically that's more of a spectrogram, though). The Wigner distribution can be applied to any Fourier pairs. It was first applied to the momentum/space conjugates of quantum mechanics (see Heisenberg uncertainty principle).

For more info, see the writeup Fourier transforms, Wigner distributions, and Wavelet transforms.

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