The extreme range, or magnitude, of a fluctuating value such as an acoustic or electromagnetic signal, amplitude is measured perpendicular to the to the time axis of a time-plot, i.e., frequency, of a sine wave. Amplitude is a measure of the intensity, loudness, power, strength, or volume level of a signal. In an electrical circuit operating on alternating current (ac), amplitude is measured as the Voltage (V) level and is expressed as +V and V, depending on the direction of the current. See also sine wave.