Definitions for "Rest mass"
the mass of an object, measured by an observer at rest with respect to the object.
(physics) the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body
The rest mass () of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by 2. When particle physicists use the word ``mass,'' they always mean the ``rest mass'' () of the object in question. The total energy of a free particle is given by where is the momentum of the particle. Note that for =0 this simplifies to Einstein's famous E=mc 2. For a general particle with mass and momentum, it can also be written as m c 2 where Some textbooks on special relativity identify γ as the ``mass'' of a moving γ particle; this definition is not used in particle physics. The quantity includes both the mass energy and the kinetic energy.