Root Mean Square. A method of averaging, most often used in the theatre when discussing DMMs in measuring voltage and current. See Learn Root Mean Square(RMS)/Quadratic Mean(QM) tutorial, definition, example, formula. RMS is most applicable when describing or measuring the power contained in an AC waveform that is not a pure sine wave. A common place in the theatre where this occurs is on the output of a phase-control dimmer. Likewise, the current drawn by a phase-control dimmer is non-sinusoidal, or nonlinear. A true-RMS meter must be used for accurately measuring both voltage and current on a phase-control dimmer. "Crest factor" is a measure of just how non-sinusoidal a particular waveform is.