an electric current that is continually varying in value and reversing its direction of flow at regular intervals, usually in a sinusoidal manner. Each repetition, from zero to a maximum in one direction and then to maximum in the other direction and back to zero, is called a cycle. The number of cycles occurring in 1 s is called a cycle. The number of cycles occurring in 1 s is called the frequency in hertz. The average value of an alternating current is zero. For alternating current terms, see AC entries.