the Bohr magneton (symbol μ_B) is a physical constant and the natural unit for expressing an electron magnetic dipole moment. The Bohr magneton is defined in SI units by

and in Gaussian CGS units by

where

e is the elementary charge,

? is the reduced Planck constant,

m_e is the electron rest mass and

c is the speed of light.

The magnitude of an electron's spin magnetic moment is approximately one Bohr magneton.