This is a constant for the electron's g factor, the characteristic proportionality constant that relates the electron's magnetic moment, μe, to the electron's angular momentum quantum number using the fundamental quantum unit of magnetism.
The g-factor, gd, (also the dimensionless magnetic moment) characterizes the magnetic moment and gyromagnetic ratio of a particle or nucleus.
This constant is specified with a standard uncertainty (standard deviation) of: 0.00000000000053