This constant is the ratio of `mu_d / mu_e`, the ratio of the masses of deuteron and an electron.  This ratio of two masses is, of course, unit-less.

Notes

Deuterium (symbol D or 2H, also known as heavy hydrogen) is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in 6,420 of hydrogen.

This constant, `mu_d/mu_e`, is specified with a standard uncertainty (standard deviation) of: 0.0000015