In a direct current (DC) circuit, voltage or current is simple to define, but in an alternating current (AC) circuit, the definition is more complicated, and can be done in several ways. Root-mean-square (rms) refersto the most common mathematical method of defining the effective voltage or current of an AC wave. To determine rms value, three mathematical operations are carried out onthe function representing the AC waveform: (1) The square of the waveform function (usually a sine wave) is determined.