This is a constant representing the proton's g factor, the characteristic proportionality constant that relates the proton's magnetic moment, μp, to the proton's angular momentum quantum number using the fundamental quantum unit of magnetism.
The g-factor, gp, (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: 0.000 000 046