Voltage or potential difference between two points in an electric circuit is1V if 1J (Joule) of energy is expended in transferring 1 C of charge between those points.
It is generally represented by the symbol V and measured in volts (V). Note that the symbol and the unit of voltage are both denoted by the same letter; however, it rarely causes any confusion.
The symbol V also signifies a constant voltage (DC) whereas a time-varying (AC) voltage is represented by the symbol v or v (t)