Planck length

Short definition from Wiktionary

Noun

1. 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: ℓ_P=√hbar Gc³≈1.616 252(81)×10⁻³⁵ mbox m