Electron-Volts are a unit of energy equal to that attained by an electron falling unimpeded through a potential difference of one volt. One electron-Volt is equal to 1.602×10-19 Joules. It is commonly abbreviated as eV and standard prefixes are used to denote larger or smaller quantities, e.g. 1 MeV = 1 million electron-volts.
The units is commonly used in atomic, nuclear and particle physics due to the small energies encountered. For example, the ionization energy of hydrogen is 13.6 eV and the energy of a typical alpha particle is 2-10 MeV.