Alternating current

Alternating current (often abbreviated as AC) refers to the voltage (and thus current) pattern used in one of the two main methods of carrying power (the other being so-called direct current, or DC).

In AC, The voltage starts at zero, increases to a positive peak, levels off, returns to zero, then goes negative, reaches a nagative peak, levels off again, and then returns to zero; the cycle then repeats the sinusoidal pattern.

AC is preferred over DC for power transmission since i) resistive losses are reduced when power is transmitted at high voltages (since resistive losses go as I^2*R), and ii) transformers are simple, passive devices which can easily convert high voltages down to the safer low voltages better for actual use and consumption - but transformers only work with AC.