Alternating current (AC) is a type of electrical current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. AC is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions, fans, and electric lamps into a wall socket.

AC is used for electrical energy distribution because AC voltage may be increased or decreased with a transformer, allowing the power to be transmitted through power lines efficiently at high voltage, which reduces the energy lost as heat due to resistance of the wire, and transformed to a lower, safer voltage for use. The use of a higher voltage leads to significantly more efficient transmission of power.

AC can be produced using a device called an alternator, which is a special type of electrical generator designed to produce alternating current. AC is also capable of powering electric motors, which is useful for many large appliances like dishwashers, refrigerators, and so on.

In contrast, DC is electrical current that flows consistently in one direction. A common source of DC power is a battery cell in a flashlight.

In summary, AC is a type of electrical current that periodically reverses direction and changes its magnitude continuously with time, while DC flows only in one direction. AC is used for electrical energy distribution and can be produced using an alternator.