The ability of a substance to allow electrical field line to pass through them is called electric permeability or permitivity.
The permeability of free space (a vacuum ) is a physical constant equal to approximately 1.257 x 10 -6 henry per meter . It is symbolized µ o . Permeability in general is symbolized µ, and is a constant of proportionality that exists between magnetic flux density and magnetic field strength in a given medium