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RMS, or Root Mean Square, is a measure of the power of a signal, such as an AC waveform. It is the effective value of the waveform, meaning the constant DC power that would deliver the same amount of energy to a resistive load. It is calculated by squaring the value of each data point in the waveform, averaging those squares, and then taking the square root of the average. This gives the RMS value, which is always less than or equal to the peak value of the waveform. For a sine wave, the RMS value is 1/root(2) or about 0.707 times the peak value.