This constant represents the quantum mechanical effect contributed to the magnetic moment of a muon.

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.  See Muon Mass.

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 63 E-3