RMS

RMS, short for root mean square, is a way of stating the effective value of a voltage or current that changes over time. An RMS voltage delivers the same heating power to a resistor as a steady DC voltage of the same number. For a sine wave the RMS value is about 0.707 times the peak, while for a square wave it equals the peak.

Example: A household outlet rated 120 volts RMS actually swings to a peak of about 170 volts, but heats a toaster as if it were a steady 120 volts.