electron volt true MS This term was made obsolete because it was redundant with the Unit Ontology term electron volt (UO:0000266). MS:1000137 OBSOLETE A non-SI unit of energy (eV) defined as the energy acquired by a particle containing one unit of charge through a potential difference of one volt. An electron-volt is equal to 1.602 176 53(14) x 10^-19 J.