Alternating current
Alternating current, or AC, is electric current that keeps reversing direction, flowing one way and then the other many times per second. Household outlets supply AC at 50 or 60 full cycles per second (hertz), depending on the country. Audio signals sent to a speaker are also a form of AC.
Example: The power from a wall outlet in North America is alternating current that goes through 60 full back-and-forth cycles each second.