A TeV is a teraelectronvolt, i.e., it is 1012 electron volts, or 1,000,000,000,000 eV. An electronvolt is the amount energy gained or lost by an electron when it is accelerated by a electrical potential of one volt.
It requires 13.6 eV to remove the electron in a hydrogen atom to infinity.
The eV is used interchangeably as a unit of energy and mass, and often particles are described as having a mass in eV; for example, an electron has a mass of 0.511 MeV and a proton around 930 MeV.
High-energy particle accelerators use TeV.