What characteristic defines alternating current (AC)?

The defining characteristic of alternating current (AC) is that the current frequently reverses direction. In AC systems, the flow of electric charge changes its direction periodically, which means that the electric current alternates in direction over time. This is in contrast to direct current (DC), where the current flows consistently in one direction.

The ability for AC to reverse direction is why it can be easily transformed into different voltage levels using transformers, making it highly effective for power distribution in homes and industries. This characteristic also allows AC to be transmitted over long distances with less energy loss compared to DC.

Considering the other aspects, the current flowing in one direction describes DC rather than AC. Furthermore, while the voltage level in AC systems can vary in a sinusoidal pattern, it is not accurate to say it does not vary at all, and stating that the current cannot be transformed is incorrect, as one of the key advantages of AC is that it can be transformed to different voltages. Thus, the frequent reversal of current in AC is what distinctly identifies it from other current types.