We're accustomed to the idea that what starts as potential energy can convert to kinetic energy, or vice versa, but as Einstein pointed out there are actually three terms in the total energy expression... what is the third?

where Ε is the total energy of the system, and is
equal to *Mc ^{2}* if the system is at rest,

Standard energy unit in atomic and subatomic physics: the
eV, which is 1.6 × 10^{-19} Joules.