- the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body.
- the mass of an object that is at rest relative to an observer. It is the mass used in Newtonian mechanics
- The physical mass of a body when it is regarded as being at rest.
- In Special and General Relativity, the observed mass of a body that is not in motion with respect to the observer. Also called invariant mass Compare relativistic mass.