This constant is the ratio of `mu_d / mu_p`, the ratio of the masses of deuteron and a proton. This ratio of two masses is, of course, unit-less.
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: 0.00000000018