quantitykind:RestMass
The $\textit{Rest Mass}$, the invariant mass, intrinsic mass, proper mass, or (in the case of bound systems or objects observed in their center of momentum frame) simply mass, is a characteristic of the total energy and momentum of an object or a system of objects that is the same in all frames of reference related by Lorentz transformations. The mass of a particle type X (electron, proton or neutron) when that particle is at rest. For an electron: $m_e = 9,109 382 15(45) 10^{-31} kg$, for a proton: $m_p = 1,672 621 637(83) 10^{-27} kg$, for a neutron: $m_n = 1,674 927 211(84) 10^{-27} kg$. Rest mass is often denoted $m_0$.