Alternating current, or AC, is electric current that changes direction repeatedly. The voltage also changes over time, usually in a smooth wave pattern in power systems.
Where AC is used
AC is the standard form of electricity delivered by many power grids and wall outlets. It is useful for long-distance distribution because it can be stepped up or down efficiently with a transformer.
AC versus DC
Direct current (DC) flows in one direction. Many electronic devices internally use DC, even when they are plugged into an AC outlet through a power adapter.
In simple terms, AC is the kind of electricity that keeps reversing direction on a schedule.