The rest mass (m) of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word "mass," they always mean the "rest mass" (m) of the object in question.
(Also called proper mass, invariant mass, velocity-independent mass.) The mass of an object measured in a coordinate system in which the object is at rest. To retain within special relativity the classical form of Newton's dynamical law of motion for a body with momentum and velocity acted upon by a force momentum may be written as The relativistic mass is defined as where is the rest mass and is the free-space speed of light. For meteorological applications, rest mass and relativistic mass are so nearly identical that it is largely pointless to make a distinction between the two. Helliwell, T. M., 1966: Introduction to Special Relativity, 148–150.