rest mass

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The physical mass of a body as measured by an observer who perceives the body as motionless. Rest mass is an inherent property of elementary particles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics the mass of a body when it is not moving relative to an observer

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (physics) the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body