Spectrogram

A Spectrogram is a 2D plot of time vs frequency for a 1D signal, where the colour (or brightness) at each point shows how much energy there is at that frequency at that time.

It's usually computed by taking the Fourier Transform of short overlapping windows of the signal (the Short-Time Fourier Transform). Where a Spectrum shows the frequency content at one point in time or across the whole signal, a spectrogram shows how it changes over time.

See also Mel Spectrogram.