A unit of energy, abbreviated eV, equal to the work requires to move one electron through a potential difference of 1 volt. The eV is a small unit of energy, so mega-electron volts (MeV) and giga-electron volts (GeV) are commonly encountered. The masses of elementary particles are frequently expressed in term of electron volts by making use of Einstein's famous equation , where m is the mass of the particle and c is the speed of light.