A commonly encountered constant in the physics of electic fields and movement of charge, Epsilon Nought (written as the greek letter Epsilon with a subscript zero) is the electrical permittivity of free space. Its value is approximately 8.854 x 10^{-12} C^{2}/N.m^{2}

As may be expected from the interrelated nature of electricity and magnetism, it has a magnetic counterpart, the permeability of free space, designated as Mu Nought. The relationship between the two is made elegantly clear by one of Maxwell's electromagnetism equations:

The speed of light = 1/ the square root of Epsilon Nought times Mu Nought.