the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body.
Compare relativistic mass.
Origin of rest mass
First recorded in 1910–15
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the mass of an object that is at rest relative to an observer. It is the mass used in Newtonian mechanics
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
The physical mass of a body when it is regarded as being at rest.
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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.
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