Alternating current is the standard current in North America that is transmitted with a positive and negative half cycle. There are 60 cycles per second.
A schematic of an AC cycle is shown below. This is one cycle of a sine wave showing the positive and negative half cycle. There are 60 of these cycles generated per second in North America. This is sometimes described as a frequency of 60 hertz.
Reference: RWMA - Resistance Welding Manual 4th Edition