Define RMS value.
RMS Value of Sinusoidal AC Waveform: The RMS value is the Root Mean Square value. It is defined as the square root of the mean value of the squares of the alternating quantity over one cycle.
And/OR
For an alternating current, the RMS value is defined as that value of steady current (DC) which produces the same power or heat as is produced by the alternating current during the same time under the same conditions.