Rest mass

The invariant mass or intrinsic mass or proper mass or just mass is a measurement or calculation of the mass of an object that is the same for all frames of reference. For any frame of reference, the invariant mass may be determined from a calculation involving an object's total energy and momentum.
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The term mass in special relativity is commonly used by physicists to mean a quantity that does not depend on the observer or the inertial frame used to observe it. However, because the term relativistic mass is also used, this occasionally leads to confusion. The invariant mass of an object (also known as the rest mass, intrinsic mass or proper mass) is an observer-independent quantity that is synonymous with mass. On the other hand, the relativistic mass of an object (also known as apparent mass) increases with its speed and therefore depends on one's frame of reference. The concept of relativistic mass has gradually fallen into disuse in physics since 1950, when particle physics showed the relevance of invariant mass, to the point that relativistic mass is rarely used in 2007 scientific research literature. However, relativity text books of the early 1920s, written by well-respected physicists, made the term "relativistic mass" common in popular discussions and even in textbooks currently in use.
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Noun
1. (physics) the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body
(hypernym) mass
(classification) physics, physical science, natural philosophy

The mass of a particle when it is not moving. The faster an object travels the greater its mass, a phenomenon called mass dilation effect. This property is imperceptible at most velocities until they reach a value close to the speed of light, known as relativistic speeds. It is commonplace for some sub-atomic particles (e.g. cosmic rays) to be moving at these speeds, however, and hence to have increased masses. In the description of a fundamental particle the rest mass is always quoted; often it is more convenient to use the relativistic speeds. It is commonplace for some sub-atomic particles (e.g. cosmic rays) to be moving at these speeds, however, and hence to have increased masses. In the description of a fundamental particle the rest mass is always quoted; often it is more convenient to use the rest mass energy which is the value in electron volts if the mass annihilates. Thus the rest mass of the electron, 9.1 x 10-31 kg, is 511 keV.

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Eng: rest mass
Urdu: سکونی کمیت ۔ اضافیتی نظریے کے مطابق ۔ اس میں وہ کمیت حرکی شامل نہیں ۔ جو وہ حرکت میں آکر حاصل کر لیتی ہے ۔