An Etymological Dictionary of Astronomy and Astrophysics

1. The unit of mass in Planck’s system of physical units, m_P = √ (ħ c/ G) = 2.176 × 10^-8 kg. It is also the mass of a → black hole whose → Compton wavelength is comparable to its → Schwarzschild radius.
   

2. The fundamental unit of mass in the theory of gravitation. It is the mass of hypothetical particles where gravitational attraction for eachother would be as strong as the electric force between two electrons separated by the same distance (Steven Weinberg).

See also: → Planck; → mass.