VAC stands for volts (electrical pressure) of alternating current (AC). The standard voltage available from a wall socket in the United States is 110 to 120 volts.
The listed AC voltage is an average of the actual voltage, meaning that 120 volts AC actually alternates from zero to 170 volts in one direction, and reverses back to zero and up to 170 volts in the other direction. This particular average is calculated using the RMS method, which stands for root-mean-square, and represents the equivalent power that 120 volts direct current (DC) delivers. This alternating current cycle occurs 60 times per second with electricity in the United States. Alternating current in Europe and most of Asia cycles 50 times per second.