Planck length

Etymology
Named after, who defined it.

Noun

 * 1)  A unit of length, believed to be the scale at which quantum gravity effects become significant, that is defined in terms of the speed of light, the gravitational constant and the reduced Planck's constant, namely: $$\ell_P =\sqrt\frac{\hbar G}{c^3} \approx 1.616 252 (81) \times 10^{-35} \mbox{ meters}$$