This usually refers to one complete set of "alternations" in alternating current (AC).

It may help to think of the cycling of alternating current as similar to the cycle of a sound wave as it appears on an oscilloscope, and in many respects the two cycles are very similar. (In fact, the 60Hz frequency of AC current is often audible as a low buzz known as "60-cycle hum".) Alternating current uses two conductors, both of which start at zero voltage. The voltage in one conductor will rapidly rise to maximum voltage and fall again to zero, and when the power is "clean", the rise and fall resembles the smooth rise and fall of a sine wave on an oscilloscope. At that point, the voltage "alternates" to the second conductor, where it rises to maximum and falls again to zero just as it did with the first conductor. This is the equivalent of an oscilloscope's wave falling below zero to its minimum and rising again to the baseline. When both conductors have gone through one rise to maximum voltage and fall to zero again, they have completed a single cycle.

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