Spectral flux

Spectral flux is a measure of how quickly the power spectrum of a signal is changing, calculated by comparing the power spectrum for one frame against the power spectrum from the previous frame.
More precisely, it is usually calculated as the L2-norm between the two normalised spectra. Calculated this way, the spectral flux is not dependent upon overall power, nor on phase considerations.
The spectral flux can be used to determine the timbre of an audio signal, or in onset detection, among other things.

Variations

Some implementations use the L1-norm rather than the L2-norm.
Some implementations do not normalise the spectra.
For onset detection, increases in energy are important, so some algorithms only include values calculated from bins in which the energy is increasing.