Short for gigaelectronvolt, or 109 eV. Is actually an amount of energy, since a volt has units of energy/charge, and the electron charge (quite obviously) has units of charge... multiply 'em, and you've got just plain energy. Is equal to 1.6022e-10 joules, quite a small amount of energy by everyday standards, but when an elementary particle has energy on the order of a GeV, it's sizeable.