This constant represents the Compton Wavelength of a neutron divided by 2 `pi`. This reduced Compton Wavelength is a common representation of mass for quantum mechanics applications.

The Compton wavelength is a quantum mechanical property of nuclear/atomic particles that is equivalent to the wavelength of a photon having the same wavelength as the rest mass of the particle.  So, in this case, the Compton Wavelength of a neutron represents the wavelength of a neutron's rest mass.

Notes

This constant is specified with a standard uncertainty (standard deviation) of: 0.000 000 000 17 E-15 m