The electronvolt is a unit of energy:
The electronvolt (eV) is strictly a unit of energy, defined as the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of 1 V.
However, in physics, it is also widely used as a unit of mass. Because of Einstein's famous mass-energy equivalence formula, energy and mass are two sides of the same coin. Strictly speaking; m = eV/c2 but in many branches of physics — like particle physics — scientists simplify calculations by using a system where the speed of light (c) is set to 1. In this system, mass and energy are considered interchangeable, and the c2 is dropped, allowing mass to also be quoted in eV directly.
Thus eV is used in particle physics and quantum mechanics to describe the incredibly small masses of subatomic particles, such as electrons or protons. For example, the mass of an electron is often cited as 0.511 MeV, while a proton's mass is about 938 MeV.
