Power is a physical measure of an audio signal, proportional to the square of its amplitude. One can also specify or measure a physical sound’s total power flux through a fixed area in space, which also varies continuously. Power may be averaged over a period of time; such an average is referred to as RMS (root mean square) power.
[Rossing 02] Thomas Rossing, et al., The Science of Sound, third edition. Addison-Wesley, 2002.