rest mass

noun

, Physics.

1. the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body.

rest mass

noun

1. the mass of an object that is at rest relative to an observer. It is the mass used in Newtonian mechanics

rest mass

1. In Special and General Relativity, the observed mass of a body that is not in motion with respect to the observer.
2. Also called invariant mass
3. Compare relativistic mass

Word History and Origins

Origin of rest mass¹

First recorded in 1910–15

Example Sentences

Consequently, its accretion disk glows brightly, radiating away about a tenth of its rest mass and exceeding by orders of magnitude the total luminosity from stars in its host galaxy.

From Scientific American

Nuclear processes could not account for their energy output, because they release only a percent fraction of their rest mass as pure energy.

From BBC

Electrons have no known substructure; they’re defined solely by their rest mass, spin, and charge, which are the same for every electron.

From Scientific American

It carries energy proportional to the radiation frequency but has zero rest mass.

From Scientific American

And all along we’ve not quite managed to pin down their actual stationary mass, although it must be small, probably less than a millionth of the rest mass of an electron.

From Scientific American