Electronvolt

The electronvolt, written $\text{eV}$, is a unit of energy equal to $1.6\times10^{-19}$ J. It is defined as the energy acquired by a particle with elementary charge $e$ when it crosses an electric potential difference of $1\text{ V}$. Its SI multiples are quite common: $\text{keV}$, $\text{MeV}$, $\text{GeV}$, and $\text{TeV}$.

The electronvolt typically doubles as a unit of mass. According to the mass-energy equivalence $E=mc^{2}$, the electronvolt can be interpreted as a unit of mass:

Though tiny, this scale of mass/energy is quite common in nuclear and particle physics. Since most processes in this domain rely on relativistic calculations where energy is frequently exchanged with mass, using electronvolts to refer to rest mass ends up being much more convenient. For instance, the mass of the proton is $m_{p}=938 \text{ MeV}/c^{2}$.

Note that for electronvolts to measure mass they must be weighed by $c^{2}$. As such, natural units are quite useful, in which $c\equiv1$. In these units, the $c^{2}$ disappears and mass and energy become truly equivalent. Then, mass can be written in $\text{eV}$, e.g. $m_{p}=938\text{ MeV}$.