The permeability of free space, μ0, is a physical constant used often in electromagnetism. It is taken to have the exact value of 4π x 10-7 N/A2 (newtons per ampere squared). It is connected to the energy stored in a magnetic field, see Hyperphysics for specific equations.
It is related to the speed of light by the equation
Visit Hyperphysics for more information on this constant.