The electron volt (eV) is a unit of energy, much smaller than a joule or erg, i.e., about 1.6 × 10-12 erg. It is the amount of energy necessary to move an electron through an electric field to a point where the field has an electric potential one volt higher.
The electron volt is commonly used for atomic-scale physics, and for the energy of individual photons. The photon energy of electromagnetic radiation (EMR) is directly proportional to its frequency, and is commonly used when citing the characteristics of very high frequency/short wavelength EMR: X-rays and gamma rays. It is analogously used for energy differences necessary for atomic excitation or ionization. It is also used for particles, both for their kinetic energy (e.g., for cosmic rays), and for their rest mass (according to the mass/energy equivalence, e=mc², i.e., 1.782662×1036 kg).
The electron volt is also used as a unit for other quantities, such as a large unit of temperature, used for plasma, corresponding to 1.160451812×104 K (as per the equation e=kBT).