Definition of alternating current

Recall that direct current has a polarity, or direction, that stays the same over a long period of time. Although the magnitude might vary—the number of amperes, volts, or watts can fluctuate—the charge carriers always flow in the same direction through the circuit.
In alternating current, the polarity reverses again and again at regular intervals. The magnitude usually changes because of this constant reversal of polarity, although there are certain cases where the magnitude doesn’t change even though the polarity does.
The rate of change of polarity is the third variable that makes ac so much different from dc. The behavior of an ac wave depends largely on this rate: the frequency.