A common symbol for the voltage in an alternating current (AC) circuit (see: var below for comments on alternating current). The "SI" does not allow symbols to be modified with additional information; instead of "12 VAC," write "AC 12 V."
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}