The change of potential energy experienced by an electron moving from a place where the potential has a value of V to a place where it has a value of (V+1 volt).
This is a convenient energy unit when dealing with the motions of electrons and ions in electric fields; the unit is also the one used to describe the energy of X-rays and gamma rays.
A keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal to one million electron volts. A GeV is equal to one billion (109) electron volts. A TeV is equal to a million million (1012) electron volts.