Alternating current (AC) is an electric current that periodically reverses direction. In an AC circuit, the flow of electric charge periodically changes direction, typically in a sinusoidal waveform. This means that the voltage and current alternate between positive and negative polarities over time. 

AC is the type of electricity commonly used in residential and commercial buildings, as well as in most electrical power generation and transmission systems. It's favored for its ability to efficiently transmit power over long distances using transformers, and it can be easily converted to different voltage levels using transformers, making it suitable for a wide range of applications.

The frequency of the alternation, measured in hertz (Hz), determines how quickly the current changes direction. In many regions, the standard frequency of AC power is 50 or 60 Hz.