The Volt is a unit for measuring electric potential. The unit is named for Alessandro Volta, an Italian physicist. Unlike the unit for electricity (ampere) it is not an SI unit. On data sheets the volt is represented through a capital V. In physical equations voltage is represented by the letter U.
VAC stands for Voltage in Alternating Current. VAC describes a periodic switch in the polarity in equal shares in the negative and positive range. The average over time for the alternating current is thus nil. A typical example of a sine wave-shaped alternating current is the 230V domestic power network.