An electron volt (eV) is the amount of kinetic energy gained by an electron as it passes through an electric potential difference of 1 Volt. It has the value:

1 eV = 1.602 176 53(14)x10^-19 Joules

with the measurement error in the decimal places in brackets. Given its small value, energies are often expressed as mega-electron volts (1 MeV = 10⁶ eV) and giga-electron volts (1 GeV = 10⁹ eV).
Although not a standard unit of measurement, it is typically used to measure the energies of elementary particles. In addition, since mass is related to energy through Einstein’s famous equation, E = mc², the masses of elementary particles are also often expressed in electron volts. For example: the mass of an electron is 0.51 MeV/ c², where c is the speed of light.