A volt is defined as the voltage that's produced when a current of one ampere dissipates one watt of power. This quantity is also equal to one joule of energy per coulomb of charge.
The volt is a derived SI unit, meaning that it's derived from SI base units. The volt may be expressed in base units as (meters squared x kilograms) / (seconds cubed x amperes). This expression may be abbreviated as m^2 x kg x s^-3 x A^-1.
The volt also may be expressed with a variety of other units. For example, a volt is also equal to one watt per ampere (W/A) and one joule per ampere per second (J/A/s).
The volt may be measured in the laboratory by using a value based on the Josephson constant. This value was established as 4.835979 x 10^14 Hertz per volt (Hz/V) by the 18th General Conference on Weights and Measures in 1990.
Voltage may be described by comparing the flow of electricity to the flow of water in a pipe. In this analogy, voltage is the pressure differential between the entrance and exit of the pipe.