Rest mass (physics), the mass of a body as measured when the body is at rest, but it is relative to an observer who is moving or not, an inherent property of the body. All matter, such as any object, has some rest mass. Energy, such as light or kinetic energy (the energy of a moving object) has no rest mass because it causes no change in spacetime in a frozen time frame. However, energy affects mass while time passes; therefore, it affects spacetime, so it has mass.