This constant represents the mass/energy equivalent of a muon and is expressed in Joules (J).
A muon is an elementary particle classified as a lepton which has properties similar to an electron: it has spin of -1/2 and charge of -1. The muon however has significantly more mass than an electron.
This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 086 E-11 J.
The concept of mass-energy equivalence asserts that the mass of an object is a measure of that object's energy and that if you add energy to the object, you will increase the mass of the object.
It would take approximately 90 megajoules of energy added to an object to increases the object's mass by 1 microgram.
The object's property called energy is an equivalent property to the object's mass and both properties are related by the equation: E=m⋅c2. This mass–energy was first defined by Einstein's theory of special relativity.