Rest Mass

Rest mass, abbreviated as \(m_0\), also called invariant mass, of a body is measured when the body is at rest and motionless, and is also relative to an observer moving or not moving.  This is the mass that an observer would measure for the particle if they were at rest relative to it.  Rest mass is a property of matter that is independent of the particle's motion, and is a fundamental property of the particle itself.  It is a key concept in Einstein's theory of relativity, where it is used to describe the relationship between energy and mass.