At least, RMS as a formulae, Root Mean Square...
Lets say the output is a time varying function F(t), then the RMS of that function Frms(t) is:
Frms(t) = Sqrt( Sqr( F(t) ) )
If you sum that, or integrate for an continuous signal (ie audio), over one period, you can use that to calculate power output. P = V^2/R. All that the squaring and rooting does, is to ensure that it's always a positive number. Mathematically Frms(t) = |F(t)| is the same, at least for a discreet functions.
**broken link removed**
Still, thought, as has been mentioned, it is BS without mentioning distortion. One such figure is THD - Total Harmonic Distortion, thought I have no idea what it's definition is...