The , 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: , for a proton: , for a neutron: . Rest mass is often denoted .
The , 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: , for a proton: , for a neutron: . Rest mass is often denoted .